Difference between revisions of "Section of an infix operator"
From HaskellWiki
(sectioning examples taken from article Currying) 

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−  In Haskell there is a special syntax for [[partial application]] on [[infix operator]]s. 
+  In Haskell there is a special syntax for [[partial application]] on [[infix operator]]s. Essentially, you only give one of the arguments to the infix operator, and it represents a function which intuitively takes an argument and puts it on the "missing" side of the infix operator. 
−  * <hask>(2^)</hask> is equivalent to <hask>(^) 2</hask> 
+  * <hask>(2^)</hask> (left section) is equivalent to <hask>(^) 2</hask>, or more verbosely <hask>\x > 2 ^ x</hask> 
−  * <hask>(^2)</hask> is equivalent to <hask>flip (^) 2</hask> 
+  * <hask>(^2)</hask> (right section) is equivalent to <hask>flip (^) 2</hask>, or more verbosely <hask>\x > x ^ 2</hask> 
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* <hask>('\t':)</hask> is the "indent" function, 
* <hask>('\t':)</hask> is the "indent" function, 

* <hask>(`elem` "AEIOU")</hask> is the "iscapitalvowelinEnglish" function (ignoring the "sometimes Y"). 
* <hask>(`elem` "AEIOU")</hask> is the "iscapitalvowelinEnglish" function (ignoring the "sometimes Y"). 

+  
+  Note: as an exception, the "" (subtraction) operator cannot do a right section, because that would be interpreted as unary negation in Haskell syntax. The Prelude function "subtract" is provided for this purpose. Instead of <hask>( e)</hask>, you need to write <hask>(subtract e)</hask>. 

== See also == 
== See also == 

* [[Currying]] 
* [[Currying]] 

−  
+  * [http://www.haskell.org/onlinereport/exps.html#sections Haskell report: Sections]  see for more details 

[[Category:Glossary]] 
[[Category:Glossary]] 
Revision as of 21:40, 1 February 2009
In Haskell there is a special syntax for partial application on infix operators. Essentially, you only give one of the arguments to the infix operator, and it represents a function which intuitively takes an argument and puts it on the "missing" side of the infix operator.

(2^)
(left section) is equivalent to(^) 2
, or more verbosely\x > 2 ^ x

(^2)
(right section) is equivalent toflip (^) 2
, or more verbosely\x > x ^ 2
Like partial application and lambda abstraction, sectioning provides a convenient way of writing some functions without having to explicitly name them:

(1+)
(unsugared:(+) 1
) is the "increment" function, 
(2*)
is the "double" function, 
('\t':)
is the "indent" function, 
(`elem` "AEIOU")
is the "iscapitalvowelinEnglish" function (ignoring the "sometimes Y").
Note: as an exception, the "" (subtraction) operator cannot do a right section, because that would be interpreted as unary negation in Haskell syntax. The Prelude function "subtract" is provided for this purpose. Instead of ( e)
, you need to write (subtract e)
.
See also
 Currying
 Haskell report: Sections  see for more details