| 97 | 97 |
|
| 98 | 98 |
(a | b) | c = a | (b | c) = a | b | c
|
| 99 | 99 |
|
|
100 |
### Similar things
|
|
101 |
|
| 100 | 102 |
It's all a bit like `MonadPlus` in Haskell, with concatenation
|
| 101 | 103 |
being a lot like `>=>`, although I really had `Either` more
|
| 102 | 104 |
in mind than `Maybe`, but `Either` isn't an instance of
|
| 103 | 105 |
`MonadPlus` for technical reasons, and I don't understand
|
| 104 | |
monads anyway I'm sure.
|
|
106 |
monads anyway I'm sure. I'd explain further, but it's strictly
|
|
107 |
taboo -- I might start babbling about burritos, you see.
|
|
108 |
|
|
109 |
Probably a more familiar thing that it's similar to is the
|
|
110 |
Bourne shell. If `a` and `b` are executables, then `a && b`
|
|
111 |
executes `a` and checks the error code. If the error code is
|
|
112 |
non-zero, it exits with that exit code, otherwise it executes
|
|
113 |
`b` and exits with its exit code. Alternately, `a || b`
|
|
114 |
executes `a` and checks the error code. If the error code is
|
|
115 |
_zero_, it exits with that exit code, otherwise it executes
|
|
116 |
`b` and exits with its exit code.
|
| 105 | 117 |
|
| 106 | 118 |
### The practical upshot of all this
|
| 107 | 119 |
|
|
| 126 | 138 |
|
| 127 | 139 |
As an example, let's try to write a factorial function in Vinegar.
|
| 128 | 140 |
|
|
141 |
Well, first, let's get some preliminaries out of the way. Up 'til
|
|
142 |
now we've been fairly vague about the actual language. Let's pin
|
|
143 |
down some concrete syntax.
|
|
144 |
|
|
145 |
-> Tests for functionality "Execute Vinegar Program"
|
|
146 |
|
|
147 |
-> Functionality "Execute Vinegar Program" is implemented by
|
|
148 |
-> shell command
|
|
149 |
-> "python3 bin/vinegar <%(test-body-file)"
|
|
150 |
|
|
151 |
Each definition is on its own line, which is terminated by
|
|
152 |
a semicolon. The result of executing a program, is the result
|
|
153 |
of executing `main`. The form `int[n]` where _n_ is a literal
|
|
154 |
integer in decimal notation, pushes _n_ onto the stack.
|
|
155 |
|
|
156 |
main = other;
|
|
157 |
other = int[3];
|
|
158 |
==> OK([3])
|
|
159 |
|
|
160 |
There is a built-in operation to swap the top two values on the stack.
|
|
161 |
|
|
162 |
main = int[100] int[200] swap;
|
|
163 |
==> OK([200, 100])
|
|
164 |
|
|
165 |
There is a built-in operation to pop the topmost value off the stack and
|
|
166 |
discard it.
|
|
167 |
|
|
168 |
main = int[40] int[50] pop int[60];
|
|
169 |
==> OK([40, 60])
|
|
170 |
|
|
171 |
If there are not enough values on the stack for an operation, it fails
|
|
172 |
with underflow.
|
|
173 |
|
|
174 |
main = swap;
|
|
175 |
==> Failure(underflow)
|
|
176 |
|
|
177 |
There is a built-in operation to pop the topmost two values and assert
|
|
178 |
that they are equal.
|
|
179 |
|
|
180 |
main = int[5] int[5] equal | int[4];
|
|
181 |
==> OK([])
|
|
182 |
|
|
183 |
main = int[5] int[8] equal | int[4];
|
|
184 |
==> OK([4])
|
|
185 |
|
|
186 |
OK, _now_ let's try to write a factorial function in Vinegar.
|
|
187 |
|
| 129 | 188 |
fact = dup <1> gt! dup <1> sub fact mul | nop
|
| 130 | 189 |
|
| 131 | 190 |
What we have here is:
|
|
| 150 | 209 |
|
| 151 | 210 |
fact = dup <1> eq! | dup <1> sub fact mul
|
| 152 | 211 |
|
|
212 |
(Here's that factorial in test form. I need to make the
|
|
213 |
syntax consistent, I know, sorry about that. Anyway:)
|
|
214 |
|
|
215 |
main = int[5] fact;
|
|
216 |
fact = dup int[1] equal | dup int[1] sub fact mul;
|
|
217 |
==> OK([120])
|
|
218 |
|
| 153 | 219 |
Now, we take the argument and assert that it *is* 1. If
|
| 154 | 220 |
it is, we just return it. If not, we compute factorial
|
| 155 | 221 |
on it, and return that. If anything in our factorial
|