diff options
| -rw-r--r-- | standard-ml.html.markdown | 21 |
1 files changed, 15 insertions, 6 deletions
diff --git a/standard-ml.html.markdown b/standard-ml.html.markdown index 133e4f54..c9eb2a2e 100644 --- a/standard-ml.html.markdown +++ b/standard-ml.html.markdown @@ -235,17 +235,18 @@ val hmm = answer "What is the meaning of life, the universe and everything?" (* Functions can take several arguments by taking one tuples as argument: *) fun solve2 (a : real, b : real, c : real) = - ( (~b + Math.sqrt(b * b - 4.0*a*c)) / (2.0 * a), - (~b - Math.sqrt(b * b - 4.0*a*c)) / (2.0 * a) ) + ((~b + Math.sqrt(b * b - 4.0 * a * c)) / (2.0 * a), + (~b - Math.sqrt(b * b - 4.0 * a * c)) / (2.0 * a)) (* Sometimes, the same computation is carried out several times. It makes sense to save and re-use the result the first time. We can use "let-bindings": *) fun solve2 (a : real, b : real, c : real) = - let val discr = b * b - 4.0*a*c + let val discr = b * b - 4.0 * a * c val sqr = Math.sqrt discr val denom = 2.0 * a in ((~b + sqr) / denom, - (~b - sqr) / denom) end + (~b - sqr) / denom) + end (* Pattern matching is a funky part of functional programming. It is an @@ -292,6 +293,9 @@ val thermometer = val some_result = (fn x => thermometer (x - 5) ^ thermometer (x + 5)) 37 (* Here is a higher-order function that works on lists (a list combinator) *) +(* map f l + applies f to each element of l from left to right, + returning the list of results. *) val readings = [ 34, 39, 37, 38, 35, 36, 37, 37, 37 ] (* first an int list *) val opinions = List.map thermometer readings (* gives [ "Cold", "Warm", ... ] *) @@ -324,7 +328,11 @@ val n = op + (5, 5) (* n is now 10 *) (* 'op' is useful when combined with high order functions because they expect functions and not operators as arguments. Most operators are really just infix functions. *) -val sum_of_numbers = foldl op+ 0 [1,2,3,4,5] +(* foldl f init [x1, x2, ..., xn] + returns + f(xn, ...f(x2, f(x1, init))...) + or init if the list is empty. *) +val sum_of_numbers = foldl op+ 0 [1, 2, 3, 4, 5] (* Datatypes are useful for creating both simple and complex structures *) @@ -407,7 +415,8 @@ fun writePoem(filename) = let val file = TextIO.openOut(filename) val _ = TextIO.output(file, "Roses are red,\nViolets are blue.\n") val _ = TextIO.output(file, "I have a gun.\nGet in the van.\n") - in TextIO.closeOut(file) end + in TextIO.closeOut(file) + end (* Read a nice poem from a file into a list of strings *) fun readPoem(filename) = |