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The Tamsin Language Specification, version 0.5-2017.0502

This document is a work in progress.

Note that this document only specifies the behaviour of Tamsin version 0.5-2017.0502. The reference interpreter in fact supports a few more features than are listed here. Those features are listed in the Advanced Features document, and may appear in a future version of Tamsin but they are not a part of 0.5-2017.0502.

This document, plus the reference implementation tamsin, is as close to normative as we're going to come for the time being. But they are still a ways from being definitive.

-> Tests for functionality "Intepret Tamsin program"

Fundaments

A Tamsin program consists of one or more productions. A production consists of a name and a parsing rule (or just "rule" for short). Among other things, a rule may be a non-terminal, which is the name of a production, or a terminal, which is a literal string in double quotes. (A full grammar for Tamsin can be found in Appendix A.)

When run, a Tamsin program processes its input. It starts at the production named main, and evaluates its rule. A non-terminal in a rule "calls" the production of that name in the program. A terminal in a a rule expects a token identical to it to be on the input. If that expectation is met, it evaluates to that token. If not, it raises an error. The final result of evaluating a Tamsin program is sent to its output.

(If it makes it easier to think about, consider "its input" to mean "stdin", and "token" to mean "character"; so the terminal "x" is a command that either reads the character x from stdin and returns it (whence it is printed to stdout by the main program), or errors out if it read something else. Or, thinking about it from the other angle, we have here the rudiments for defining a grammar for parsing a trivial language.)

| main = blerf.
| blerf = "p".
+ p
= p

| main = blerf.
| blerf = "p".
+ k
? expected 'p' but found 'k'

Productions can be written that don't look at the input. A rule may also consist of the keyword return, followed a term; this expression simply evaluates to that term and returns it. (More on terms later; for now, think of them as strings without any quotes around them.)

So, the following program always outputs blerp, no matter what the input is.

| main = return blerp.
+ fadda wadda badda kadda nadda sadda hey
= blerp

Note that in the following, blerp refers to the production named "blerp" in one place, and in the other place, it refers to the term blerp. Tamsin sees the difference because of the context; return must be followed by a term, while a parsing rule cannot be part of a term.

| main = blerp.
| blerp = return blerp.
+ foo
+ foo
+ foo 0 0 0 0 0
= blerp

A rule may also consist of the keyword print followed by a term, which, when evaluated, sends the term to the output, and evaluates to the term. (Mostly this is useful for debugging. In the following, world is repeated because it is both printed, and the result of the evaluation.)

| main = print hello & print world.
+ ahoshoshohspohdphs
= hello
= world
= world

A rule may also consist of two subrules joined by the & operator. The & operator processes the left-hand side rule. If the LHS fails, then the & expression fails; otherwise, it continues and processes the right-hand side rule. If the RHS fails, the & expression fails; otherwise it evaluates to what the RHS evaluated to.

| main = "a" & "p".
+ ap
= p

| main = "a" & "p".
+ ak
? expected 'p' but found 'k'

| main = "a" & "p".
+ ep
? expected 'a' but found 'e'

If you are too used to C or Javascript or the shell, you may use && instead of &.

| main = "a" && "p".
+ ap
= p

A rule may also consist of two subrules joined by the | operator. The & operator processes the left-hand side rule. If the LHS succeeds, then the | expression evaluates to what the LHS evaluted to, and the RHS is ignored. But if the LHS fails, it processes the RHS; if the RHS fails, the | expression fails, but otherwise it evaluates to what the RHS evaluated to.

For example, this program accepts 0 or 1 but nothing else.

| main = "0" | "1".
+ 0
= 0

| main = "0" | "1".
+ 1
= 1

| main = "0" | "1".
+ 2
? expected '1' but found '2'

If you are too used to C or Javascript or the shell, you may use || instead of |.

| main = "0" || "1".
+ 1
= 1

Using return described above, this program accepts 0 or 1 and evaluates to the opposite. (Note here also that & has a higher precedence than |.)

| main = "0" & return 1 | "1" & return 0.
+ 0
= 1

| main = "0" & return 1 | "1" & return 0.
+ 1
= 0

| main = "0" & return 1 | "1" & return 0.
+ 2
? expected '1' but found '2'

Evaluation order can be altered by using parentheses, as per usual.

| main = "0" & ("0" | "1") & "1" & return ok.
+ 011
= ok

Note that if the LHS of | fails, the RHS is tried at the position of the stream that the LHS started on. This property is called "backtracking".

| ohone = "0" & "1".
| ohtwo = "0" & "2".
| main = ohone | ohtwo.
+ 02
= 2

Note that print and return never fail. Thus, code like the following is "useless":

| main = foo & print hi | return useless.
| foo = return bar | print useless.
= hi
= hi

Note that return does not exit the production immediately — although this behaviour may be re-considered...

| main = return hello & print not_useless.
= not_useless
= not_useless

Alternatives can select code to be executed, based on the input.

| main = aorb & print aorb | cord & print cord & return ok.
| aorb = "a" & print ay | "b" & print bee.
| cord = "c" & print see | eorf & print eorf.
| eorf = "e" & print ee | "f" & print eff.
+ e
= ee
= eorf
= cord
= ok

And that's the basics. With these tools, you can write simple recursive-descent parsers. For example, to consume nested parentheses containing a zero:

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ 0.
= ok

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ (((0))).
= ok

(the error message on this test case is a little weird; it's because of the backtracking. It tries to match (((0))) against the beginning of input, and fails, because the last ) is not present. So it tries to match 0 at the beginning instead, and fails that too.)

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ (((0)).
? expected '0' but found '('

(the error message on this one is much more reasonable...)

| main = parens & "." & return ok.
| parens = "(" & parens & ")" | "0".
+ ((0))).
? expected '.' but found ')'

To consume a comma-seperated list of one or more bits:

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 1.
= .

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,1,1,0,1,1,1,1,0,0,0,0,1.
= .

(again, backtracking makes the error a little odd)

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,,1,0.
? expected '.' but found ','

| main = bit & {"," & bit} & ".".
| bit = "0" | "1".
+ 0,10,0.
? expected '.' but found '0'

Comments

A Tamsin comment is introduced with # and continues until the end of the line.

| # welcome to my Tamsin program!
| main = # comments may appear anywhere in the syntax
|        # and a comment may be followed by a comment
|   "z".
+ z
= z

A comment can appear at the end of a Tamsin program, after everything.

| main = "z".         # and so it goes
+ z
= z

Variables

When a production is called, the result that it evaluates to may be stored in a variable. Variables are local to the production.

| main = blerp → B & blerp & "." & return B.
| blerp = "a" | "b".
+ ab.
= a

Note that you don't have to use the Unicode arrow. You can use an ASCII digraph instead.

| main = blerp -> B & blerp & "." & return B.
| blerp = "a" | "b".
+ ab.
= a

In fact, the result of not just a production, but any rule, may be sent into a variable by →. Note that → has a higher precedence than &.

| main = ("0" | "1") → B & return B.
+ 0
= 0

A → expression evaluates to the result placed in the variable.

| main = ("0" | "1") → B.
+ 0
= 0

This isn't the only way to set a variable. You can also do so unconditionally with set.

| main = eee.
| eee = set E = whatever && set F = stuff && return E.
+ ignored
= whatever

And note that variables are subject to backtracking, too; if a variable is set while parsing something that failed, it is no longer set in the | alternative.

| main = set E = original &
|          (set E = changed && "0" && "1" | "0" && "2") &
|        return E.
+ 01
= changed

| main = set E = original &
|          (set E = changed && "0" && "1" | "0" && "2") &
|        return E.
+ 02
= original

Names of Variables must be Capitalized.

| main = set b = blerp & return b.
? expected

Terms

We must now digress for a definition of Tamsin's basic data type, the term.

A term T is defined inductively as follows:

An atom, written as a character string, is a term;
A constructor, written S(T1, T2, ... Tn) where S is a character string and T1 through Tn are terms (called the subterms of T), is a term;
A variable, written as a character string where the first character is a capital Latin letter, is a term;
Nothing else is a term.

In fact, there is little theoretical difference between an atom and a constructor with zero subterms, but they are considered different things for conceptual clarity.

A term is called ground if it does not contain any variables.

Terms support an operation called expansion, which also requires a context C (a map from variable names to ground terms.)

expand(T, C) when T is an atom evaluates to T;
expand(T, C) when T is a constructor S(T1,...,Tn) evaluates to a new term S(expand(T1, C), ... expand(Tn, C));
expand(T, C) when T is a variable looks up T in C and, if there is a ground term T' associated with T in C, evaluates to T'; otherwise the result is not defined.

The result of expansion will always be a ground term.

Ground terms support an operation called flattening (also sometimes called stringification).

flatten(T) when T is an atom, results in that atom;
flatten(T) when T is a constructor S(T1,...Tn) results in an atom comprising
```
S · "(" · flatten(T1) · "," · ... · "," · flatten(Tn) · ")"
```
where · is string concatenation;

The result of flattening is always an atom.

Ground terms also support an operation called reprifying (also sometimes called "readable stringification"). It is very similar to flattening, but results in an atom, the contents of which is always a legal syntactic atom in term context in a Tamsin program. (Flattening a term does not always guarantee this because, for example, flattening '\n' results in an actual newline.)

repr(T) when T is an atom whose text consists only of one or more ASCII characters in the ranges a to z, A to Z, 0 to 9, and _, results in T;
repr(T) when T is any other atom results in an atom comprising
```
"'" · T′ · "'"
```
where T′ is T with all non-printable and non-ASCII bytes replaced by their associated \xXX escape sequences (for example, newline is \x0a), and with \ replaced by \\ and ' replaced by \';
repr(T) when T is a constructor S(T1,...Tn) whose text S consists only of one or more ASCII characters in the ranges listed above, results in
```
S · "(" · repr(T1) · "," · ... · "," · repr(Tn) · ")"
```
repr(T) when T is a any other constructor S(T1,...Tn) results in
```
"'" · S′ · "'" · "(" · repr(T1) · ", " · ... · ", " · repr(Tn) · ")"
```
where · is string concatenation and S′ is defined the same way as T′ is for atoms;

Note that in the above, "printable" means ASCII characters between 32 (space) and 126 ~. It is not dependent on locale.

Also, \xXX escapes will always be output in lowercase, e.g. \x0a, not \x0A.

The input to a Tamsin production is, in fact, an atom (although it's hardly atomic; "atom" is sort of a quaint moniker for the role these objects play.)

The contexts in Tamsin which expect a term expression are return, set, the right-hand side of → (which you haven't seen yet,) and arguments to productions (which you also haven't seen yet.) In these contexts, a bareword evaluates to an atom rather than a non-terminal.

| main = return hello.
= hello

But an atom can contain arbitrary text. To write an atom which contains spaces or other things which are not "bareword", enclose it in single quotes.

| main = return Hello, world!
? expected

| main = return 'Hello, world!'.
= Hello, world!

Note that the atom 'X' is not the same as the variable X. Nor is the atom 'tree(a,b)' the same as the constructor tree(a,b).

In a term context, a constructor may be given with parentheses after the string.

| main = return hello(world).
= hello(world)

The bareword rule applies in subterms.

| main = return hello(beautiful world).
? expected

| main = return hello('beautiful world').
= hello(beautiful world)

In a term context, variables may be given. Terms are expanded during evaluation, unless they are patterns (which we'll get to eventually.)

| main = set E = world & return hello(E).
= hello(world)

A term expression may also contain a + operator, which evaluates and flattens both its arguments and concatenates the resulting atoms.

| main = set E = world & return 'hello, ' + E + '!'.
= hello, world!

And note, underscores are allowed in production and variable names, and atoms without quotes.

| main = this_prod.
| this_prod = set Var_name = this_atom & return Var_name.
= this_atom

Escape Sequences

A literal string may contain escape sequences. Note, I hate escape sequences! So I might not leave this feature in, or, at least, not quite like this.

| main = "a" & "\"" & "b" & print 'don\'t'.
+ a"b
= don't
= don't

| main = "a" & "\\" & "b" & print 'don\\t'.
+ a\b
= don\t
= don\t

| main = "a" & "\n" & "b" & print 'don\nt'.
+ a
+ b
= don
= t
= don
= t

| main = "a" & "\t" & "b" & print 'don\tt'.
+ a b
= don   t
= don   t

The escape sequence \x must be followed by two hex digits.

| main = "a" & "\x4a" & "b" & print 'don\x4at'.
+ aJb
= donJt
= donJt

Note also that you can print a constructor.

| main = print hi(there('I\'m'(a(constructor)))).
= hi(there(I'm(a(constructor))))
= hi(there(I'm(a(constructor))))

List sugar

In a term context, [] is sugar for a list structure.

| main = return [a, b, c].
= list(a, list(b, list(c, nil)))

The tail of the list default to the atom nil, but an "improper" list can be given using the | syntax, like Prolog or Erlang.

| main = return [a, b | c].
= list(a, list(b, c))

An empty list is just nil.

| main = return [].
= nil

Only one term may appear after the |.

| main = return [a, b | c, d].
? expected

The list sugar syntax may also be used in match patterns (see far below.)

Examples using Terms

This program accepts a pair of bits and evaluates to a term, a constructor pair, with the two bits as subterms.

| main = bit → A & bit → B & return pair(A, B).
| bit = "0" | "1".
+ 10
= pair(1, 0)

| main = bit → A & bit → B & return pair(A, B).
| bit = "0" | "1".
+ 01
= pair(0, 1)

This program expects an infinite number of 0's. It will be disappointed.

| main = zeroes.
| zeroes = "0" & zeroes.
+ 00000
? expected '0' but found EOF

This program expects a finite number of 0's, and returns a term representing how many it found. It will not be disappointed.

| main = zeroes.
| zeroes = ("0" & zeroes → E & return zero(E)) | return nil.
+ 0000
= zero(zero(zero(zero(nil))))

We can also use concatenation to construct the resulting term as an atom.

| main = zeroes.
| zeroes = ("0" & zeroes → E & return E + 'Z') | return ''.
+ 0000
= ZZZZ

Implicit `set` and `return`

Unquoted atoms and constructors ("barewords") can have the same names as productions. If they are used in rule context, they are assumed to refer to productions. If they are used in term context, they are assumed to refer to terms.

| main = blerf.
| blerf = return blerf.
= blerf

Because variable names cannot be mistaken for productions, if they are used in rule context and followed by ←, this is equivalent to set.

| main = S ← blerf & "x" & return S.
+ x
= blerf

There is of course an ASCII digraph for the left-pointing arrow. (The right-pointing symbol in the input in this test is just to get keep my text editor's syntax highlighting under control.)

| main = S <- blerf & "x" & return S.
+ x->
= blerf

If the variable name is not followed by ←, this is an implied return of the variable's value.

| main = S ← blerf & "x" & S.
+ x
= blerf

If a quoted term (atom or constructor) is used in rule context, this too cannot be mistaken for a production. So this, too, implies a return of that term.

| main = S ← blerf & "x" & 'frelb'.
+ x
= frelb

(Not so sure about this one. It makes the grammar compflicated.)

# | main = S ← blerf & "x" & 'frelb'(S).
# + x
# = frelb(blerf)

But it must be quoted, or Tamsin'll think it's a production.

| main = S ← blerf & "x" & frelb.
+ x
? frelb

Aside: ← vs. →

One may well ask why Tamsin has both →, to send the result of a rule into a variable, and ←, to send a term into a variable, when both of these could be done with one symbol, in one direction, and in fact most languages do it this way (with a symbol like =, usually.)

Two reasons:

This way gives us two disjoint syntax contexts (rule context and term context) which lets us re-use the same symbols (such as lowercased barewords) for dual purposes. Which in turn lets us write more compact code.

And also, parsing is a linear process. When we consume tokens from the input, whether directly with a terminal, or indirectly via a non-terminal, we want them to be easily located. We want all our ducks to be in a row, so to speak. This setup ensures that the focus of parsing is always on the left and not nested inside a term. Like so:

| main = "(" &
|        expr → S &
|        "," &
|        expr → T &
|        U ← pair(S,T) &
|        ")" &
|        U.
| expr = "a"
|      | "b"
|      | "c".
+ (b,c)
= pair(b, c)

That said, it is possible to use only the → if you like, by using return (or implicit return!) instead of set. Like so:

| main = "(" &
|        expr → S &
|        "," &
|        expr → T &
|        return pair(S,T) → U &
|        ")" &
|        U.
| expr = "a"
|      | "b"
|      | "c".
+ (b,c)
= pair(b, c)

In my opinion, this style is not as clear, because at the rule which updates U, U itself is the focus and should be on the left.

What about the other way around? We could introduce some symbol (say, /) which allows a rule in what would otherwise be a term context, for example

main = "(" &
       S ← /expr &
       "," &
       T ← /expr &
       U ← pair(S,T) &
       ")" &
       U.
expr = "a"
     | "b"
     | "c".

This would also work, and is more similar to conventional programming languages; however, in my opinion, it is not as clear either, because in the rules which parse the sub-expressions, it is expr that is the focus of the logic, rather than the variables the results are being sent into.

Static Checking

Note that the production named by a non-terminal must exist in the program, even if it is never evaluated.

| main = "k" | something_undefined.
+ k
? something_undefined

Advanced Parsing

eof

If there is more input available than what we wrote the program to consume, the program still succeeds.

| main = "a" & "p".
+ apparently
= p

The built-in production eof may be used to match against the end of the input. If there is no more input remaining, it succeeds and returns an empty string atom.

| main = "a" & "p" & eof.
+ ap
=

But if there still is input remaining, it fails.

| main = "a" & "p" & eof.
+ apt
? expected EOF but found 't'

Note that, in both cases, eof doesn't consume anything. So if it succeeded at the end of input, and the program tries eof again, it will still succeed the second time, and time after time after that.

| main = "a" & "p" & eof & eof & eof & eof & eof & eof.
+ ap
=

any

The built-in production any matches any token defined by the scanner except for EOF. (Remember that for now "token defined by the scanner" means "character", but that that can be changed, as you'll see below.)

| main = any & any & any.
+ (@)
= )

| main = any & any.
+ a
? expected any token but found EOF

Optional rules

The rule [FOO] is a short form for (FOO | return nil).

| main = ["0"].
+ 0
= 0

| main = ["0"].
+ 
= nil

So we can rewrite the "zeroes" example to be simpler:

| main = zeroes.
| zeroes = ["0" & zeroes → E & return zero(E)].
+ 0000
= zero(zero(zero(zero(nil))))

Iterated rules

The rule {FOO} is what it is in EBNF, and/or a while loop. Like [], we don't strictly need it, because we could just write it as recursive BNF. But it's handy. Like while loops are handy. It returns the result of the last successful rule applied, or nil if none were successful.

| main = {"0"}.
+ 0 0 0 0
= 0

| main = {"0"}.
+ 1 2 3 4
= nil

Backtracking applies to {} too.

| zeroesone = {"0"} & "1".
| zeroestwo = {"0"} & "2".
| main = zeroesone | zeroestwo.
+ 000002
= 2

So we can rewrite the "zeroes" example to be even... I hesistate to use the word "simpler", but we can... write it differently.

| main = zeroes.
| zeroes = set Z = nil & {"0" && set Z = zero(Z)} & return Z.
+ 0000
= zero(zero(zero(zero(nil))))

fail

The built-in production fail always fails. This lets you establish global flags, of a sort. It takes a term, which it uses as the failure message.

| debug = return ok.
| main = (debug & return walla | "0").
+ 0
= walla

| debug = fail notdebugging.
| main = (debug & return walla | "0").
+ 0
= 0

| main = set E = 'Goodbye, world!' & fail E.
+ hsihdsihdsih
? Goodbye, world!

!

The ! ("not") keyword is followed by a rule. If the rule succeeds, the ! expression fails. If the rule fails, the ! expression succeeds. In neither case is input consumed — anything the rule matched, is backtracked. Thus ! should almost always be followed by & and something which consumes input, such as any.

| main = !"k" & any.
+ l
= l

| main = !"k" & any.
+ k
? expected anything else

| main = !("k" | "r") & any.
+ l
= l

| main = !("k" | "r") & any.
+ k
? expected anything else

| main = !("k" | "r") & any.
+ r
? expected anything else

This is particularly useful for parsing strings and comments and anything that contains arbitrary text terminated by a sentinel.

| main = "'" & T ← '' & {!"'" & any → S & T ← T + S} & "'" & return T.
+ 'any bloody
+   gobbledegook *!^*(^@)(@* (*@#(*^*(^(!^
+ you like.'
= any bloody
=   gobbledegook *!^*(^@)(@* (*@#(*^*(^(!^
= you like.

Multi-terminal sugar

The syntax “abc” (with the proper, oriented, opening and closing quotation marks) is syntactic sugar for "a" & "b" & "c" & return 'abc'.

| main = “appar”.
+ apparently
= appar

| main = “appar”.
+ apple
? expected 'a' but found 'l'

This is useful for writing scanners in Tamsin that are based on the $:byte or $:utf8 scanner, but which return multi-character tokens. (See "Advanced Scanning", below.)

Dynamic Terminals

As mentioned, the terminal "foo" matches a literal token foo in the buffer. But what if you want to match something dynamic, something you have in a variable? You can do that with «»:

| main = set E = f & «E».
+ f
= f

| main = set E = f & «E».
+ b
? expected 'f' but found 'b'

Note that you don't have to use the Latin-1 guillemets. You can use the ASCII digraphs instead.

| main = set E = f & <<E>>.
+ b
? expected 'f' but found 'b'

Terms are flattened for use in «». So in fact, the "foo" syntax is just syntactic sugar for «'foo'».

| main = «'f'».
+ f
= f

Oh, and since we were speaking of sentinels earlier...

| main = {sentineled → A & print A & {" "}} & return ok.
| sentineled =
|    "(" &
|    any → S &
|    T ← '' & {!«S» & any → A & T ← T + A} & «S» &
|    ")" &
|    T.
+ (!do let's ))) put &c. in this string!)   (&and!this!one&)
= do let's ))) put &c. in this string
= and!this!one
= ok

folds

The following idiom is essentially a fold from functional programming.

| main = T ← '' & {$:alnum → S & T ← T + S} & return T.
+ dogwood
= dogwood

It is so common, that Tamsin supports a special form for it. The infix operator / takes a rule on the left-hand side, and a term (used as the initial value) on the right-hand side, and expands to the above.

| main = $:alnum/''.
+ dogwood
= dogwood

| main = $:alnum/'prefix'.
+ dogwood.
= prefixdogwood

You can use any rule you desire, not just a non-terminal, on the LHS of /.

| main = ("0" | "1")/'%'.
+ 0110110110.
= %0110110110

Note that the RHS of / is a term expression, so it can contain a +.

| main = ("0" | "1")/'%' + '&'.
+ 0110110110.
= %&0110110110

If there is an additional /, it must be followed by an atom. This atom will be used as a constructor, instead of the concat operation.

| main = $:alnum/nil/cons.
+ dog.
= cons(g, cons(o, cons(d, nil)))

Note that the middle of // is a term expression.

| main = $:alnum/cat+food/cons.
+ dog.
= cons(g, cons(o, cons(d, catfood)))

Note that the RHS of // is not a term expression.

| main = $:alnum/ni+l/co+ns.
+ dog.
? expected

Not that (for now) /'s cannot be nested. But you can make a sub-production for this purpose.

| main = ("*" & string)/nil/cons.
| string = $:alnum/''.
+ *hi*there*nice*day*isnt*it
= cons(it, cons(isnt, cons(day, cons(nice, cons(there, cons(hi, nil))))))

Modules

This section needs to be written. Basically, a module is a set of productions inside a namespace. There is one built-in module called $ and it is always in scope.

System Module

The module $ contains a number of built-in productions which would not be possible or practical to implement in Tamsin. See Appendix C for a list.

In fact, we have been using the $ module already! But our usage of it has been hidden under some syntactic sugar. For example, "k" is actually...

| main = $:expect(k).
+ k
= k

| main = $:expect(k).
+ l
? expected 'k' but found 'l'

The section about aliases needs to be written too.

Details of the system module can be found in System_Module.markdown.

Modules in General

:foo always means production foo in the current module.

| main = :blah.
| blah = "b" & print 'hello'.
+ b
= hello
= hello

So, you can name your own productions the same as built-in keywords, as long as you call them with :foo.

| main = :set.
| set = :return.
| return = :fail.
| fail = :print.
| print = :any.
| any = :eof.
| eof = "x".
+ x
= x

Defining a Module

Here is the syntax for defining a module:

| blah {
|   expr = "y".
| }
| main = expr.
| expr = "x".
+ x
= x

| blah {
|   expr = "y".
| }
| main = blah:expr.
| expr = "x".
+ y
= y

| blah {
|   expr = blah:goo.
|   goo = "y".
| }
| main = blah:expr & blah:goo & "@".
| expr = "x".
+ yy@
= @

:foo (and indeed foo) refers to the production foo in the same module as the production where it's called from.

| blah {
|   expr = :goo.
|   goo = "y".
| }
| main = blah:expr.
| goo = "x".
+ y
= y

| foo {
|   expr = goo.
|   goo = "6".
| }
| bar {
|   expr = goo.
|   goo = "4".
| }
| main = foo:goo & bar:goo.
+ 64
= 4

Can't call a production or a module that doesn't exist.

| foo {
|   expr = goo.
|   goo = "6".
| }
| main = foo:zoo.
? zoo

| foo {
|   expr = goo.
|   goo = "6".
| }
| main = zoo.
? zoo

| foo {
|   expr = goo.
|   goo = "6".
| }
| main = boo:zoo.
? boo

You can have a Tamsin program that is all modules and no productions, but you can't run it.

| foo {
|   main = "6".
| }
? main

Evaluation

Arguments to Productions

A production may be called with arguments.

| main = blerf(world).
| blerf(X) = return hello(X).
= hello(world)

No variables from the caller leak into the called production.

| main = set FizzBuzzWhatever = whatever & donkey(world).
| donkey(E) = return hello(FizzBuzzWhatever).
? FizzBuzzWhatever

Note that this makes the «»-form more interesting.

| main = bracketed(a) & bracketed(b) & return ok.
| bracketed(X) = «X» & "S" & «X».
+ aSabSb
= ok

| main = bracketed(a) & bracketed(b) & return ok.
| bracketed(X) = «X» & "S" & «X».
+ aSabSa
? expected 'b' but found 'a'

We need to be able to test arguments somehow. We can do that with pattern-matching, which works in Tamsin very similarly to how it works in Erlang (except here, there are no guards or list sugar.)

| main = blerf(tree(a, b)).
| blerf(tree(X, Y)) = return X.
= a

| main = blerf(c).
| blerf(a) = return zzrk.
| blerf(b) = return zon.
| blerf(c) = return zzt.
= zzt

| main = blerf(d).
| blerf(a) = return zzrk.
| blerf(b) = return zon.
| blerf(c) = return zzt.
? No 'blerf' production matched arguments

Thus, we can write productions that recursively call themselves, and terminate on the base case.

| main = blerf(tree(tree(tree(a,b),c),d)).
| blerf(tree(L,R)) = blerf(L).
| blerf(Other) = return Other.
= a

What does this get us? Functional programming! Let's parse a tree, then return the rightmost bottommost leaf.

| main = tree → T & rightmost(T).
| tree = "t" & "r" & "e" & "e" &
|        "(" & tree → L & "," & tree → R & ")" & return tree(L, R)
|      | "0" | "1" | "2".
| rightmost(tree(L,R)) = rightmost(R).
| rightmost(X) = return X.
+ tree(tree(0,1),tree(0,tree(1,2)))
= 2

Note that + is part of a "term expression", but cannot be used as a pattern.

| main = what(hel+lo).
| what(he+llo) = 'yes'.
? expected

Note that parentheses can only be given in a production call when there are arguments to pass. If there are no arguments, there should be no parentheses.

| main = what().
| what = "2".
? expected

| main = what.
| what() = "2".
? expected

Note that the list sugar syntax can also be used in patterns.

| main = expr([a, b, c]) & 'ok'.
| expr([]) = print 'end'.
| expr([H|T]) = print H & expr(T).
= a
= b
= c
= end
= ok

Advanced Scanning

Implicit Buffer

Object-oriented languages sometimes have an "implicit self". That means when you say just foo, it's (generally) assumed to refer to a method or field on the current object that is in context.

Tamsin, clearly, has an implicit buffer. This is the buffer on which scanning/parsing operations like terminals operate. When you call another production from a production, that production you call gets the same implicit buffer you were working on. And main gets its implicit buffer from some implementation-defined place (in the reference interpreter, it gets its from Python's idea of "standard input" to the program.)

So, also clearly, there should be some way to alter the implicit buffer when you call another production. And there is.

The syntax for this is postfix @, because you're pointing the production "at" some other text...

| main = set T = 't(a,t(b,c))' & tree @ T.
| tree = "t" & "(" & tree → L & "," & tree → R & ")" & return fwee(L, R)
|      | "a" | "b" | "c".
+ doesn't matter
= fwee(a, fwee(b, c))

This is a good way to process atoms in Tamsin.

| main = print_each_char @ 'Hello'.
| print_each_char = any → C & print C & print_each_char | return 'ok'.
+ doesn't matter
= H
= e
= l
= l
= o
= ok

The term doesn't have to be an atom. The term expression will be flattened.

| main = print_each_char @ f(b).
| print_each_char = any → C & print C & print_each_char | return 'ok'.
+ doesn't matter
= f
= (
= b
= )
= ok

This can be wrapped up to make the term an argument to a production call:

| main = print_each_char(fo+ob+ar).
| print_each_char(X) = print_each_char_r @ X.
| print_each_char_r = any → C & print C & print_each_char_r | return 'ok'.
+ doesn't matter
= f
= o
= o
= b
= a
= r
= ok

The rule being applied to the specified buffer doesn't have to be a non-terminal, either. It can be any rule (but watch the precedence.)

| main = $:alnum @ 'Hi!'.
= H

| main = {$:alnum} @ 'Hi!'.
= i

@'s nest.

| main = one @ 'I process this string until ! where I digress a bit' & ''.
| one = {"!" & two @ 'Here I digress' | any → C & $:emit(C)}.
| two = {any → C & $:emit(C)}.
= I process this string until Here I digress where I digress a bit

Implicit Scanner

Actually, the implicit buffer is just one component of the implicit scanner that is in effect at any given point in a Tamsin program. Not only may its buffer be changed, but its scanning rules, and thus the set of tokens it returns, may be changed as well.

As you have seen, the default scanner returns single characters.

| main = "a" & "b" & "c".
+ abc
= c

| main = "abc".
+ abc
? expected 'abc' but found 'a'

You can select a different scanner for a rule with using. There are two built-in scanners in the built-in $ module that you can use: $:utf8, which consumes Unicode characters encoded in UTF-8 (and which is the default scanner for a Tamsin program), and $:byte, which consumes raw bytes.

| main = ("a" & "b" & "c") using $:utf8.
+ abc
= c

| main = "abc" using $:utf8.
+ abc
? expected 'abc' but found 'a'

| main = ("«" | "♡")/''.
+ «♡««♡←
= «♡««♡

| main = {"«" | "♡"} & eof.
+ «♡«→«♡
? expected EOF but found '→'

Here we test the $:byte scanner...

| main = ("a" & "b" & "c") using $:byte.
+ abc
= c

| main = "abc" using $:byte.
+ abc
? expected 'abc' but found 'a'

-> Tests for functionality "Intepret Tamsin program (pre- & post-processed)"

The byte scanner is 8-bit clean. (The 0a added to the output is the newline.)

| main = (any & any & any) using $:byte.
+ 010003
= 030a

This includes bytes that would be special in UTF-8.

| main = (any & any → R & any & R) using $:byte.
+ 00ff00
= ff0a

| main = "\x00" → N using $:byte & return '\x01' + N + '\xff'.
+ 00
= 0100ff0a

| main = ("\x07" & ("\xf0" | "\xfa")/'' → N & "\x07") using $:byte & N.
+ 07f0fafaf0f007
= f0fafaf0f00a

-> Tests for functionality "Intepret Tamsin program"

Defining a custom scanner

You can also define your own scanner by defining a production designed to return tokens. Each time it is called, it should return an atom, which the user of your scanner will see as a scanned token.

When you name a production in the program with using, that production should return a token each time it is called. We call this scanner a "production-defined scanner" or just "production scanner". In the following, we use a production scanner based on the scanner production.

We'll use the following scanner in the next few examples. It accepts only the tokens cat and dog, and no other symbols. Note that we are not using it yet in this example; this example just demonstrates that the token production returns tokens.

| main = {token → A & print A} & 'ok'.
| token = ("c" & "a" & "t" & 'cat' | "d" & "o" & "g" & 'dog') using $:utf8.
+ catdogdogcatcatdog
= cat
= dog
= dog
= cat
= cat
= dog
= ok

Here's a slightly more practical scanner that we'll also use in the next few examples.

| main = {token → A & print A} & 'ok'.
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ cabbage( bag     gaffe fad ) ()) bag(bagbag bag)
= cabbage
= (
= bag
= gaffe
= fad
= )
= (
= )
= )
= bag
= (
= bagbag
= bag
= )
= ok

Here is how you would use the above scanner, as a scanner, in a program:

| main = ("(" & "cons" & ")" & 'ok') using token.
| 
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ ( cons )
= ok

| main = ("(" & "cons" & ")" & 'ok') using token.
| 
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ ( quote )
? expected 'cons' but found 'quote'

Note that, if your scanner-production doesn't itself say what scanner it is using, it defaults to the $:utf8 scanner.

| main = ("(" & "cons" & ")" & 'ok') using token.
| 
| token = {" "} & ("(" | ")" | word).
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ ( cons )
= ok

Note that while it's conventional for a production scanner to return terms similar to the strings it scanned, this is just a convention, and may be subverted:

| main = ("meow" & "woof") using token.
| token = ("c" & "a" & "t" & 'meow' | "d" & "o" & "g" & 'woof').
+ catdog
= woof

If a production scanner fails to match the input text, it will signal an EOF. The justification for this is that it's the end of the input, as far as the scanner can understand it.

| main = program using scanner.
| scanner = "a" | "b" | "@".
| program = "a" & "@" & "b" & return ok.
+ x
? expected 'a' but found EOF

If you don't like that, you can write your scanner to fail the way you want.

| main = program using scanner.
| scanner = "a" | "b" | "@" | return bleah.
| program = "a" & "@" & "b" & return ok.
+ x
? expected 'a' but found 'bleah'

On the other hand, if the scanner understands all the tokens, but the parser doesn't see the tokens it expects, you get the usual error.

| main = program using scanner.
| scanner = "a" | "b" | "@".
| program = "a" & "@" & "b" & return ok.
+ b@a
? expected 'a' but found 'b'

Parsing using a production scanner ignores any extra text given to it, just like the built-in parser.

| main = program using scanner.
| scanner = (
|             "c" & "a" & "t" & return cat | "d" & "o" & "g" & return dog
|           ).
| program = "cat" & "dog".
+ catdogfoobar
= dog

The production scanner properly handles backtracking on a per-token basis.

| main = program using scanner.
| scanner = (
|             "c" & "a" & "t" & return cat | "d" & "o" & "g" & return dog
|           ).
| program = "cat" & print 1 &
|           ("cat" & print 2 | "dog" & print 3) &
|           "dog" & print 4 & return ok.
+ catcatdog
= 1
= 2
= 4
= ok

| main = program using scanner.
| scanner = (
|             "c" & "a" & "t" & return cat | "d" & "o" & "g" & return dog
|           ).
| program = "cat" & print 1 &
|           ("cat" & print 2 | "dog" & print 3) &
|           "dog" & print 4 & return ok.
+ catdogdog
= 1
= 3
= 4
= ok

You can mix two scanners in one production.

| main = "dog" using token & ("!" & "!" & "!") using $:utf8 & return ok.
| 
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ dog!!!
= ok

Note that the token scanner we've defined doesn't consume spaces after a token, and that the char scanner doesn't skip spaces.

| main = "dog" using token & ("c" & "a" & "t") using $:utf8 & return ok.
| 
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ dog cat
? expected 'c' but found ' '

But we can convince it to skip those spaces...

| main = "dog" using token
|      & ({" "} & "c" & "a" & "t") using $:utf8
|      & return ok.
| 
| token = ({" "} & ("(" | ")" | word)) using $:utf8.
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ dog        cat
= ok

Note that the scanner in force is lexically contained in the using. Outside of the using, scanning returns to whatever scanner was in force before the using.

| main = "dog" using token
|      & ({"."} & "c" & "a" & "t")
|      & return ok.
| 
| token = ({" "} & ("(" | ")" | word)).
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ dog...........cat
= ok

On the other hand, variables set when one scanner is in effect can be accessed by rules with another scanner in effect, as long as they're in the same production.

| main = ("c" & "a" & "t" → G) using $:utf8
|      & ("dog" & return G) using token.
| 
| token = ({" "} & ("(" | ")" | word)).
| word = $:alnum → L & {$:alnum → M & set L = L + M} & L.
+ cat dog
= t

Note: you need to be careful when using using! Beware putting using inside a rule that can fail, i.e. the LHS of | or inside a {}. Because if it does fail and the interpreter reverts the scanner to its previous state, its previous state may have been with a different scanning logic. The result may well be eurr.

(Actually, I don't know if it's possible to shoot yourself in the foot with it too badly anymore. But it used to be. But I think most of those cases are handled more nicely now.)

Advanced use of `using`

A production scanner may contain an embedded using and use another production scanner.

| main = program using scan1.
| 
| scan1 = "a" | "b" | "c" | "(" & other & ")" & return list.
| 
| other = xyz using scan2.
| xyz = "1" & "1" | "1" & "2" | "2" & "3".
| 
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & "list" & "a".
+ c(xx)a
= a

| main = program using scan1.
| 
| scan1 = "a" | "b" | "c" | "(" & other & ")" & return list.
| 
| other = xyz using scan2.
| xyz = "1" & "1" | "1" & "2" | "2" & "3".
| 
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & "list" & "a".
+ c(yy)a
? expected 'list' but found EOF

Maybe an excessive number of minor variations on that...

| main = program using scanner1.
| 
| scanner1 = scan1 using $:utf8.
| scan1 = "a" | "b" | "c" | "(" & xyz using scanner2 & ")" & return list.
| 
| xyz = "1" & "1" | "1" & "2" | "2" & "3".
| 
| scanner2 = scan2 using $:utf8.
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & "list" & "a".
+ c(xx)a
= a

| main = program using scanner1.
| 
| scanner1 = scan1 using $:utf8.
| scan1 = "a" | "b" | "c" | "(" & {other} & ")" & return list.
| 
| other = xyz using scanner2.
| xyz = "1" & "1" | "1" & "2" | "2" & "3".
| 
| scanner2 = scan2 using $:utf8.
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & "list" & "a".
+ c(xxxyyzxy)a
= a

| main = program using scanner1.
| 
| scanner1 = scan1 using $:utf8.
| scan1 = "a" | "b" | "c" | "(" & {xyz using scanner2} & ")" & return list.
| 
| xyz = "1" & "1" | "1" & "2" | "2" & "3".
| 
| scanner2 = scan2 using $:utf8.
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & "list" & "a".
+ c(xxxyyzxy)a
= a

| main = program using scanner1.
| 
| scanner1 = scan1 using $:utf8.
| scan1 = "a" | "b" | "c"
|       | "(" & {xyz → R using scanner2} & ")" & return R.
| 
| xyz = "1" & "1" & return 11 | "1" & "2" & return 12 | "2" & "3" & return 23.
| 
| scanner2 = scan2 using $:utf8.
| scan2 = "x" & return 1 | "y" & return 2 | "z" & return 3.
| program = "c" & ("11" | "12" | "23") → R & "a" & return R.
+ c(xxxyyzxy)a
= 12

The production being applied with the production scanner can also switch its own scanner. It switches back to the production scanner when done.

| main = program using scanner.
| 
| scanner = {" "} & set T = '' & {("a" | "b" | "c") → S & set T = T + S}.
| 
| program = "abc" & "cba" & "bac".
+ abc    cba bac
= bac

| main = program using scanner.
| 
| scanner = {" "} & set T = '' & {("a" | "b" | "c") → S & set T = T + S}.
| 
| program = "abc" & (subprogram using subscanner) & "bac".
| 
| subscanner = {" "} & set T = '' & {("s" | "t" | "u") → S & set T = T + S}.
| 
| subprogram = "stu" & "uuu".
+ abc    stu   uuu bac
= bac

Combining `using` and `@`

It is entirely possible to do so.

| main = {any → T using scan & print T} & 'ok'.
| scan = S ← '' & {$:alnum → C & S ← S + C} & {" " | "," | "."} & return S.
+ This, this is my string.
= This
= this
= is
= my
= string
= ok

| main = {any → T using scan & print T} @ 'This, this is my string.' & 'ok'.
| scan = S ← '' & {$:alnum → C & S ← S + C} & {" " | "," | "."} & return S.
= This
= this
= is
= my
= string
= ok

Implementation-Defined Matters

This specification intentionally leaves some things undefined. The reference implementation chooses to do these things a certain way, but this choice should not be regarded as normative. Other implementations may do them a different way, and still claim to be implementations of the Tamsin language.

These things are:

where the input to the implicit buffer that the main production works on comes from
where the final result of evaluating a Tamsin program goes to
what external modules are available to the program
how and when external modules are loaded
how external modules are stored externally and located for loading

The reference interpreter tamsin chooses the following:

input comes from Python's idea of standard input
the final result is printed to Python's idea of standard output
external modules... are not yet supported

This all implies that some external modules are optional, and that a good chunk of $ may in fact be optional, and those productions might be moved to a different module, or $ itself may in fact be optional.

Appendix A. Grammar

Here we give an approximation of Tamsin's grammar, in EBNF. Note however that this is non-normative; the canonical grammar definition for Tamsin is written in Tamsin and can be found in eg/tamsin-parser.tamsin.

Grammar    ::= {"@" Pragma "."} Production {Production "."}.
Production ::= ProdName ["(" Term {"," Term} ")" | "[" Expr0 "]"] "=" Expr0.
Expr0      ::= Expr1 {("||" | "|") Expr1}.
Expr1      ::= Expr2 {("&&" | "&") Expr2}.
Expr2      ::= Expr3 ["using" ProdRef].
Expr3      ::= Expr4 [("→" | "->") Term].
Expr4      ::= Expr5 ["/" Texpr ["/" Term)]].
Expr4      ::= "(" Expr0 ")"
             | "[" Expr0 "]"
             | "{" Expr0 "}"
             | "!" Expr0
             | "set" Variable "=" Texpr
             | "return" Texpr
             | "fail" Texpr
             | Terminal
             | Variable [("←" | "<-") Texpr]
             | ProdRef ["(" [Texpr {"," Texpr}] ")"] ["@" Texpr].
Texpr      ::= Term {"+" Term}.
Term       ::= Atom ["(" Term {"," Term} ")"]
             | Variable.
Terminal   ::= DoubleQuotedStringLiteral
             | ("«" | "<<") Texpr ("»" | ">>").
ProdRef    ::= [[ModuleRef] ":"] ProdName.
ModuleRef  ::= "$" | ModName.
Pragma     ::= "alias" ProdName Integer "=" ProdRef
             | "unalias" ProdName.
Atom       ::= ("'" {any} "'" | { "a".."z" | "0".."9" }) using $.char.
Variable   ::= ("A".."Z" { "a".."z" | "0".."9" }) using $.char.
ProdName   ::= { "a".."z" | "0".."9" } using $.char.

Appendix B. Tamsin Scanner

This section is non-normative. The canonical definition of Tamsin's scanner is written in Tamsin and can be found in eg/tamsin-scanner.tamsin.

The Tamsin scanner is designed to be relatively simple and predictable. One property in particular is that the token returned by this scanner is identical to the token that is scanned. (For example, & and && represent the same operator; thus the Tamsin scanner could return & for both of them, or even something more abstract like OP_SEQUENCE. But it doesn't; it returns && for && and & for &.

    | main = ("&&" → S & "&" → T & 'pair'(S,T))
    |        using tamsin_scanner:scanner.
    + &&&
    = pair(&&, &)

There is one exception to this rule: escape codes in literal strings are expanded in the scanner. Note that in the following, it is not repr'ed to '"\\n"'.

    | main = ($:startswith('"') → S & $:repr(S))
    |        using tamsin_scanner:scanner.
    + "\n"
    = '"\n"'

The original design of Tamsin had it expose the Tamsin scanner (for use with using) as $.tamsin. However, this may not be desirable for all implementations (e.g the compiler-to-C), and the Tamsin scanner has since been implemented in Tamsin itself (see eg/tamsin-scanner.tamsin.) Therefore $.tamsin no longer exists.

Appendix C. System Module

$:alnum — succeeds only on token which begins with alphanumeric
$:any — fails on eof, succeeds and returns token on any other token
$:byte — 8-bit-clean byte scanner production
$:eof — succeeds on eof and returns eof, otherwise fails
$:equal(L,R) — succeeds if L and R are identical terms, otherwise fails
$:expect(X) — succeeds if token is X and returns it, otherwise fails
$:fail(X) — always fails, giving X as the error message
$:mkterm(A,L) — given an atom and a list, return a single constructor
$:not(X) — succeeds only if token is not X or EOF, and returns token
$:print(X) — prints X to output as a side-effect, returns X
$:repr(X) — returns an atom which is the reprified version of C
$:return(X) — always succeeds, returning X
$:reverse(X, T) — returns the reverse of the list X, with tail of T
$:startswith(X) — consumes token if it starts with first character of X
$:unquote(X,L,R) — consumes nothing; returns X without quotes if X is quoted
$:utf8 — UTF-8-encoded Unicode character scanner production