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diff --git a/content/html b/content/html new file mode 160000 +Subproject 27ed46c9129db86669052d4e7211340da1081d8 diff --git a/content/html/gemfeed/2010-04-09-standard-ml-and-haskell.html b/content/html/gemfeed/2010-04-09-standard-ml-and-haskell.html deleted file mode 100644 index 307c3593..00000000 --- a/content/html/gemfeed/2010-04-09-standard-ml-and-haskell.html +++ /dev/null @@ -1,192 +0,0 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> -<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"> -<head> -<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> -<title>Standard ML and Haskell</title> -<link rel="shortcut icon" type="image/gif" href="/favicon.ico" /> -<style type="text/css"> -body { - margin: auto; - max-width: 900px; - background-color: #FFFFEF; - border: 1px dashed #880000; - border-radius: 8px; - padding: 5px; -} -img { - display:block; - max-width: 80%; -} -a.textlink:before { - content: " ⇒ "; - padding-left: 2px; -} -a.textlink { - text-decoration: none; - color: #FF0000; -} -a.textlink:hover { - text-decoration: underline; -} -i { - color: #48AAAD; -} -pre { - background-color: #F1F8E9; - border: 1px dashed #BB0000; - border-radius: 8px; - padding: 5px; - font-family: "Lucida Console", "Courier New", monospace; -} -h1 { - text-align: center; - color: #880000; -} -h2, h3 { - color: #BB0000; -} -</style> -</head> -<body> -<h1>Standard ML and Haskell</h1> -<p class="quote"><i>Written by Paul Buetow 2010-04-09</i></p> -<p>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.</p> -<p>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. </p> -<p>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:</p> -<h2>Defining a multi data type</h2> -<p>Standard ML:</p> -<pre> -datatype ’a multi - = EMPTY - | ELEM of ’a - | UNION of ’a multi * ’a multi -</pre> -<p>Haskell:</p> -<pre> -data (Eq a) => Multi a - = Empty - | Elem a - | Union (Multi a) (Multi a) - deriving Show -</pre> -<h2>Processing a multi</h2> -<p>Standard ML:</p> -<pre> -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 -</pre> -<p>Haskell:</p> -<pre> -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 -</pre> -<h2>Simplify function</h2> -<p>Standard ML:</p> -<pre> -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 -</pre> -<p>Haskell:</p> -<pre> -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 -</pre> -<h2>Delete all</h2> -<p>Standard ML:</p> -<pre> -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 -</pre> -<p>Haskell:</p> -<pre> -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 -</pre> -<h2>Delete one</h2> -<p>Standard ML:</p> -<pre> -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 -</pre> -<p>Haskell:</p> -<pre> -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) -</pre> -<h2>Higher order functions</h2> -<p>The first line is always the SML code, the second line always the Haskell variant:</p> -<pre> -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 -</pre> -<p>E-Mail me your thoughts at comments@mx.buetow.org!</p> -<a class="textlink" href="../">Go back to the main site</a><br /> -</body> -</html> |