Section of an infix operator

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Revision as of 14:04, 3 July 2007 (edit) Lemming (Talk | contribs) (sectioning examples taken from article Currying) ← Previous diff		Current revision (21:40, 1 February 2009) (edit) (undo) Newacct (Talk | contribs)
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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 "is-capital-vowel-in-English" function (ignoring the "sometimes Y").		* <hask>(`elem` "AEIOU")</hask> is the "is-capital-vowel-in-English" 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]]
	[[Category:Syntax]]		[[Category:Syntax]]

---

Current revision

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 to
  flip (^) 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 "is-capital-vowel-in-English" 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 , you need to write .

See also

• Currying
• Haskell report: Sections - see for more details

Categories: Glossary | Syntax