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diff --git a/content/gemtext b/content/gemtext new file mode 160000 +Subproject faa2cd392016af9b76a4f40eeb12003785ced15 diff --git a/content/gemtext/gemfeed/2010-04-09-standard-ml-and-haskell.gmi b/content/gemtext/gemfeed/2010-04-09-standard-ml-and-haskell.gmi deleted file mode 100644 index a4a1dc57..00000000 --- a/content/gemtext/gemfeed/2010-04-09-standard-ml-and-haskell.gmi +++ /dev/null @@ -1,174 +0,0 @@ -# Standard ML and Haskell - -> Written by Paul Buetow 2010-04-09 - -I am currently looking into the functional programming language Standard ML (aka SML). The purpose is to refresh my functional programming skills and to learn something new too. Since I already know a little Haskell, could I do not help myself and I implemented the same exercises in Haskell too. - -As you will see, SML and Haskell are very similar (at least when it comes to the basics). However, the syntax of Haskell is a bit more "advanced". Haskell utilizes fewer keywords (e.g. no val, end, fun, fn ...). Haskell also allows to explicitly write down the function types. What I have been missing in SML so far is the so-called pattern guards. Although this is a very superficial comparison for now, so far I like Haskell more than SML. Nevertheless, I thought it would be fun to demonstrate a few simple functions of both languages to show off the similarities. - -Haskell is also a "pure functional" programming language, whereas SML also makes explicit use of imperative concepts. I am by far not a specialist in either of these languages but here are a few functions implemented in both, SML and Haskell: - -## Defining a multi data type - -Standard ML: - -``` -datatype ’a multi - = EMPTY - | ELEM of ’a - | UNION of ’a multi * ’a multi -``` - -Haskell: - -``` -data (Eq a) => Multi a - = Empty - | Elem a - | Union (Multi a) (Multi a) - deriving Show -``` - -## Processing a multi - -Standard ML: - -``` -fun number (EMPTY) _ = 0 - | number (ELEM x) w = if x = w then 1 else 0 - | number (UNION (x,y)) w = (number x w) + (number y w) -fun test_number w = number (UNION (EMPTY, \ - UNION (ELEM 4, UNION (ELEM 6, \ - UNION (UNION (ELEM 4, ELEM 4), EMPTY))))) w -``` - -Haskell: - -``` -number Empty _ = 0 -number (Elem x) w = if x == w then 1 else 0 -test_number w = number (Union Empty \ - (Union (Elem 4) (Union (Elem 6) \ - (Union (Union (Elem 4) (Elem 4)) Empty)))) w -``` - -## Simplify function - -Standard ML: - -``` -fun simplify (UNION (x,y)) = - let fun is_empty (EMPTY) = true | is_empty _ = false - val x’ = simplify x - val y’ = simplify y - in if (is_empty x’) andalso (is_empty y’) - then EMPTY - else if (is_empty x’) - then y’ - else if (is_empty y’) - then x’ - else UNION (x’, y’) - end - | simplify x = x -``` - -Haskell: - -``` -simplify (Union x y) - | (isEmpty x’) && (isEmpty y’) = Empty - | isEmpty x’ = y’ - | isEmpty y’ = x’ - | otherwise = Union x’ y’ - where - isEmpty Empty = True - isEmpty _ = False - x’ = simplify x - y’ = simplify y -simplify x = x -``` - -## Delete all - -Standard ML: - -``` -fun delete_all m w = - let fun delete_all’ (ELEM x) = if x = w then EMPTY else ELEM x - | delete_all’ (UNION (x,y)) = UNION (delete_all’ x, delete_all’ y) - | delete_all’ x = x - in simplify (delete_all’ m) - end -``` - -Haskell: - -``` -delete_all m w = simplify (delete_all’ m) - where - delete_all’ (Elem x) = if x == w then Empty else Elem x - delete_all’ (Union x y) = Union (delete_all’ x) (delete_all’ y) - delete_all’ x = x -``` - -## Delete one - -Standard ML: - -``` -fun delete_one m w = - let fun delete_one’ (UNION (x,y)) = - let val (x’, deleted) = delete_one’ x - in if deleted - then (UNION (x’, y), deleted) - else let val (y’, deleted) = delete_one’ y - in (UNION (x, y’), deleted) - end - end - | delete_one’ (ELEM x) = - if x = w then (EMPTY, true) else (ELEM x, false) - | delete_one’ x = (x, false) - val (m’, _) = delete_one’ m - in simplify m’ - end -``` - -Haskell: - -``` -delete_one m w = do - let (m’, _) = delete_one’ m - simplify m’ - where - delete_one’ (Union x y) = - let (x’, deleted) = delete_one’ x - in if deleted - then (Union x’ y, deleted) - else let (y’, deleted) = delete_one’ y - in (Union x y’, deleted) - delete_one’ (Elem x) = - if x == w then (Empty, True) else (Elem x, False) - delete_one’ x = (x, False) -``` - -## Higher order functions - -The first line is always the SML code, the second line always the Haskell variant: - -``` -fun make_map_fn f1 = fn (x,y) => f1 x :: y -make_map_fn f1 = \x y -> f1 x : y - -fun make_filter_fn f1 = fn (x,y) => if f1 x then x :: y else y -make_filter_fn f1 = \x y -> if f1 then x : y else y - -fun my_map f l = foldr (make_map_fn f) [] l -my_map f l = foldr (make_map_fn f) [] l - -fun my_filter f l = foldr (make_filter_fn f) [] l -my_filter f l = foldr (make_filter_fn f) [] l -``` - -E-Mail me your thoughts at comments@mx.buetow.org! - -=> ../ Go back to the main site |