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.<br> | ||
| + | In the '''expression style''' you compose big expressions from small expressions. | ||
| + | |||
| + | == Comparison == | ||
As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function <hask>filter</hask>: | As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function <hask>filter</hask>: | ||
| Line 8: | Line 12: | ||
</haskell> | </haskell> | ||
| - | == Declaration style == | + | === Declaration style === |
<haskell> | <haskell> | ||
| Line 19: | Line 23: | ||
</haskell> | </haskell> | ||
| - | == Expression style == | + | === Expression style === |
<haskell> | <haskell> | ||
| Line 33: | Line 37: | ||
</haskell> | </haskell> | ||
| - | == | + | == Syntactic elements == |
There are characteristic elements of both styles. | There are characteristic elements of both styles. | ||
| - | {| | + | {| cellpadding="4" cellspacing="0" border="3" style="color:blue" |
| - | | Declaration style | + | ! style="color:black" colspan="2"|Declaration style |
| - | |- | + | ! style="color:black" colspan="2"|Expression-style |
| - | | <hask>where</hask> clause | | + | |- style="color:black" |
| - | |- | + | | colspan="2"| <hask>where</hask> clause |
| + | | colspan="2"| <hask>let</hask> expression | ||
| + | |- 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> | ||
| - | |- | + | |- 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> | ||
| - | |- | + | |- 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 == | ||
| - | + | * [[Let vs. Where]] | |
| - | * | + | * [[History of Haskell]] (in section 4.4) |
[[Category:Style]] | [[Category:Style]] | ||
[[Category:Syntax]] | [[Category:Syntax]] | ||
Current revision
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 |
1 Comparison
As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude functionfilter
filter :: (a -> Bool) -> [a] -> [a]
1.1 Declaration style
filter p [] = [] filter p (x:xs) | p x = x : rest | otherwise = rest where rest = filter p xs
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
2 Syntactic elements
There are characteristic elements of both styles.
| Declaration style | Expression-style | ||
|---|---|---|---|
| where | let | ||
| Function arguments on left hand side: | f x = x*x | Lambda abstraction: | f = \x -> x*x |
| Pattern matching in function definitions: | f [] = 0 | case | f xs = case xs of [] -> 0 |
| Guards on function definitions: | f [x] | x>0 = 'a' | if | f [x] = if x>0 then 'a' else ... |
3 See also
- Let vs. Where
- History of Haskell (in section 4.4)
