Commit a585cd2243ac28252fd43a0d47ecb6b8e34b448c - Treacle

Markdown tweaks and minor spelling/wording tweaks. catseye 11 years ago

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7	7	------------
8	8
9	9	Treacle is a programming language based on an extended form of
10		term-rewriting which we shall call, somewhat innacurately (or at least
11		arbitrarily,) context rewriting.
12
13		Like Arboretuum, its successor built around forest-rewriting, Treacle
	10	term-rewriting which we shall call, somewhat inaccurately (or at least
	11	arbitrarily,) _context rewriting_.
	12
	13	Like Arboretuum, its successor built around _forest-rewriting_, Treacle
14	14	was intended as a language for specifying compilers. Treacle is somewhat
15	15	more successful at pulling it off, however; context rewriting
16	16	encompasses, and is more expressive than, forest-rewriting.
17	17
18	18	Context rewriting is meant to refer to the fact that Treacle's rewriting
19		patterns may contain holes – designated "containers" for subpatterns
	19	patterns may contain _holes_ – designated "containers" for subpatterns
20	20	which may match not just the immediate child of the term which the
21	21	parent pattern matched (as in conventional term-rewriting) but also *any
22	22	one of that child's descendents*, no matter how deeply nested.

30	30	pattern.
31	31
32	32	Context rewriting also deconstructs the conventional concept of the
33		variable, splitting it into a name and a wildcard. Any pattern or
	33	variable, splitting it into a _name_ and a _wildcard_. Any pattern or
34	34	subpattern may be named, not just wildcards. Even holes may be named. At
35	35	the same time, wildcards, which match arbitrary terms, may occur
36	36	unnamed. Upon a successful match, only those terms which matched named
37	37	patterns are recorded in the unifier.
38	38
39		Further, each rule in Treacle may contain multiple terms (replacements)
	39	Further, each rule in Treacle may contain multiple terms (_replacements_)
40	40	on the right-hand side of a rewriting rule, and each of these may have
41		its own name. When the term undergoing rewriting (called the subject) is
	41	its own name. When the term undergoing rewriting (called the _subject_) is
42	42	rewritten, each named replacement is substituted into the subject at the
43	43	position matched by the part of the pattern that is labelled by that
44	44	same name.
45	45
46		Lastly, replacements may contain special atomic terms called newrefs.
	46	Lastly, replacements may contain special atomic terms called _newrefs_.
47	47	When a newref is written into the subject, it takes the form of a new,
48	48	unique symbol, guaranteed (or at least reasonably assumed) to be
49	49	different from all other symbols that are in, or could be in, the

82	82
83	83	Now we are ready to give some examples.
84	84
85		### Patterns
	85	### Patterns ###
86	86
87	87	- The pattern `(a b (? X *))` matches `(a b (c (d b)))`, with the
88	88	unifier `X=(c (d b))`. Also, `(a (? Y ) (c (d (? Y ))))` matches

110	110	search the contents of the 3rd subterm, for whatever the 2nd subterm
111	111	is.
112	112
113		### Rules
	113	### Rules ###
114	114
115	115	- Say we have a rule where the pattern is `(a b (:i (? X (d b))))`,
116	116	and the lone replacement is `X : a`. This rule would match the

193	193	of information in the tree of terms that relate to the reduction you
194	194	want to accomplish, they have to be in a bounded distance from, and in a
195	195	fixed relationship with, each other. If some piece is far away, it will
196		have to be brought – literally brought! – to bear on the situation, by
	196	have to be brought to bear on the situation – literally brought, by
197	197	moving it through the tree through successive "bubbling" rewrites.
198	198
199	199	Forest-rewriting eases this by having multiple independent trees: some