Difference between revisions of "Declaration vs. expression style"
From HaskellWiki
(SPJ's filter example) 
(introduction) 

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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. 

+  In the '''expression style''' you compose big expressions from small expressions. 

+  
⚫  
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>: 

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</haskell> 
</haskell> 

−  == Declaration style == 
+  === Declaration style === 
<haskell> 
<haskell> 

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</haskell> 
</haskell> 

−  == Expression style == 
+  === Expression style === 
<haskell> 
<haskell> 

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</haskell> 
</haskell> 

⚫  
+  == Syntactic elements == 

There are characteristic elements of both styles. 
There are characteristic elements of both styles. 

{ 
{ 

−   Declaration style   Expressionstyle  
+   '''Declaration style'''   '''Expressionstyle'''  
 
 

 <hask>where</hask> clause   <hask>let</hask> expression  
 <hask>where</hask> clause   <hask>let</hask> expression  
Revision as of 12:58, 3 July 2007
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.
Comparison
As illustration for the two styles, Simon Peyton Jones give two implementations of the Prelude function filter
:
filter :: (a > Bool) > [a] > [a]
Declaration style
filter p [] = []
filter p (x:xs)
 p x = x : rest
 otherwise = rest
where
rest = filter p xs
Expression style
filter =
\p > \ xs >
case xs of
[] > []
(x:xs) >
let rest = filter p xs
in if p x
then x : rest
else rest
Syntactic elements
There are characteristic elements of both styles.
Declaration style  Expressionstyle  
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 ...
