Add some Standard ML examples.
Chris Pressey
4 years ago
| 0 | Standard ML Examples | |
| 1 | ==================== | |
| 2 | ||
| 3 | -> Functionality "Evaluate Standard ML expression" | |
| 4 | -> is implemented by shell command | |
| 5 | -> "cp %(test-body-file) /tmp/foo.sml && echo ';printVal it' >>/tmp/foo.sml && mosml -quietdec /tmp/foo.sml" | |
| 6 | ||
| 7 | -> Tests for functionality "Evaluate Standard ML expression" | |
| 8 | ||
| 9 | Some basic expressions. | |
| 10 | ||
| 11 | 1 | |
| 12 | ===> 1 | |
| 13 | ||
| 14 | ~100 * ~100 * ~100 | |
| 15 | ===> ~1000000 | |
| 16 | ||
| 17 | A conditional. | |
| 18 | ||
| 19 | if true then 5 else 6 | |
| 20 | ===> 5 | |
| 21 | ||
| 22 | The basic usage of `let` to create name bindings. | |
| 23 | ||
| 24 | let val a = 2 in a end | |
| 25 | ===> 2 | |
| 26 | ||
| 27 | let fun r(x) = 77 in r(1) end | |
| 28 | ===> 77 | |
| 29 | ||
| 30 | let fun r(x) = x in r(66) end | |
| 31 | ===> 66 | |
| 32 | ||
| 33 | You can also use `val` and `fn` to define function values in a `let`. | |
| 34 | ||
| 35 | let val r = fn(x) => x in r(66) end | |
| 36 | ===> 66 | |
| 37 | ||
| 38 | ML's `let` is like Scheme's `let*`: later bindings can refer to earlier ones. | |
| 39 | ||
| 40 | let | |
| 41 | fun r(x) = x * 2 | |
| 42 | fun p(x) = r(x) * 2 | |
| 43 | in | |
| 44 | p(10) | |
| 45 | end | |
| 46 | ===> 40 | |
| 47 | ||
| 48 | However in a `let`, earlier bindings cannot refer to later ones. | |
| 49 | ||
| 50 | Skipping because I don't know why `mosml` doesn't report this error under `falderal`. | |
| 51 | It does when you run it from the command line. | |
| 52 | ||
| 53 | > main() = | |
| 54 | > let | |
| 55 | > fun oddp(x) = if x = 0 then false else evenp(x - 1) | |
| 56 | > fun evenp(x) = if x = 0 then true else oddp(x - 1) | |
| 57 | > in | |
| 58 | > evenp(6) | |
| 59 | > end | |
| 60 | > ???> Unbound value identifier: evenp | |
| 61 | ||
| 62 | You need to use `rec` in order to let earlier bindings refer to later ones, | |
| 63 | and you need to use `and` between all the mutually recursive functions. | |
| 64 | ||
| 65 | let | |
| 66 | val rec oddp = fn(x) => if x = 0 then false else evenp(x - 1) | |
| 67 | and rec evenp = fn(x) => if x = 0 then true else oddp(x - 1) | |
| 68 | in | |
| 69 | evenp(6) | |
| 70 | end | |
| 71 | ===> true | |
| 72 | ||
| 73 | let | |
| 74 | val rec oddp = fn(x) => if x = 0 then false else evenp(x - 1) | |
| 75 | and rec evenp = fn(x) => if x = 0 then true else oddp(x - 1) | |
| 76 | in | |
| 77 | evenp(7) | |
| 78 | end | |
| 79 | ===> false |