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Declaration vs. expression style

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which are both supported by Haskell mainly because several language designers preferred these different styles.
 
which are both supported by Haskell mainly because several language designers preferred these different styles.
   
In the '''declaration style''' you formulate an algorithm in terms of several equations that shall be satisfied.
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In the '''declaration style''' you formulate an algorithm in terms of several equations that shall be satisfied.<br>
 
In the '''expression style''' you compose big expressions from small expressions.
 
In the '''expression style''' you compose big expressions from small expressions.
   
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There are characteristic elements of both styles.
 
There are characteristic elements of both styles.
   
{|
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{| cellpadding="4" cellspacing="0" border="3" style="color:blue"
| '''Declaration style''' || || '''Expression-style''' ||
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! style="color:black" colspan="2"|Declaration style
|-
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! style="color:black" colspan="2"|Expression-style
| <hask>where</hask> clause || || <hask>let</hask> expression ||
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|- style="color:black"
|-
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| colspan="2"| <hask>where</hask> clause
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| colspan="2"| <hask>let</hask> expression
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|- style="color:black"
 
| Function arguments on left hand side: || <hask>f x = x*x</hask> || [[Lambda abstraction]]: || <hask>f = \x -> x*x</hask>
 
| Function arguments on left hand side: || <hask>f x = x*x</hask> || [[Lambda abstraction]]: || <hask>f = \x -> x*x</hask>
|-
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|- style="color:black"
 
| [[Pattern matching]] in function definitions: || <hask>f [] = 0</hask> || <hask>case</hask> expression: || <hask>f xs = case xs of [] -> 0</hask>
 
| [[Pattern matching]] in function definitions: || <hask>f [] = 0</hask> || <hask>case</hask> expression: || <hask>f xs = case xs of [] -> 0</hask>
|-
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|- style="color:black"
 
| [[Guard]]s on function definitions: || <hask>f [x] | x>0 = 'a'</hask> || <hask>if</hask> expression: || <hask>f [x] = if x>0 then 'a' else ...</hask>
 
| [[Guard]]s on function definitions: || <hask>f [x] | x>0 = 'a'</hask> || <hask>if</hask> expression: || <hask>f [x] = if x>0 then 'a' else ...</hask>
 
|}
 
|}
   
 
== See also ==
 
== See also ==
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* [[Let vs. Where]]
* http://research.microsoft.com/~simonpj/papers/history-of-haskell/
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* [[History of Haskell]] (in section 4.4)
   
 
[[Category:Style]]
 
[[Category:Style]]

Latest revision as of 00:41, 23 August 2008

There are two main styles of writing functional programs, which are both supported by Haskell mainly because several language designers preferred these different styles.

In the declaration style you formulate an algorithm in terms of several equations that shall be satisfied.
In the expression style you compose big expressions from small expressions.

Contents

[edit] 1 Comparison

As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function
filter
:
filter :: (a -> Bool) -> [a] -> [a]

[edit] 1.1 Declaration style

filter p [] = []
filter p (x:xs)
   | p x = x : rest
   | otherwise = rest
   where
     rest = filter p xs

[edit] 1.2 Expression style

filter =
   \p -> \ xs ->
      case xs of
         [] -> []
         (x:xs) ->
            let rest = filter p xs
            in  if p x
                  then x : rest
                  else rest

[edit] 2 Syntactic elements

There are characteristic elements of both styles.

Declaration style Expression-style
where
clause
let
expression
Function arguments on left hand side:
f x = x*x
Lambda abstraction:
f = \x -> x*x
Pattern matching in function definitions:
f [] = 0
case
expression:
f xs = case xs of [] -> 0
Guards on function definitions:
f [x] | x>0 = 'a'
if
expression:
f [x] = if x>0 then 'a' else ...

[edit] 3 See also