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Weak head normal form

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(Add a note about strict fields and note (inspired by Stack Overflow question) that the difference between builtin and defined functions is immaterial.)
 
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Note that the arguments do not themselves have to be fully evaluated for an expression to be in weak head normal form; thus, while (square 42) can be reduced to (42 * 42), which can itself be reduced to a normal form of 1764, Just (square 42) is WHNF without further evaluation. Similarly, (+) (2 * 3 * 4) is WHNF, even though (2 * 3 * 4) could be reduced to the normal form 24.
 
Note that the arguments do not themselves have to be fully evaluated for an expression to be in weak head normal form; thus, while (square 42) can be reduced to (42 * 42), which can itself be reduced to a normal form of 1764, Just (square 42) is WHNF without further evaluation. Similarly, (+) (2 * 3 * 4) is WHNF, even though (2 * 3 * 4) could be reduced to the normal form 24.
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An exception is the case of a fully applied constructor for a data type with some fields declared as strict; the arguments for these fields then also need to be in WHNF.
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The above definition might seem to treat built-in functions differently from functions defined via lambda abstraction. However, the distinction does not matter to semantics. If a lambda abstraction is applied to "too few arguments", then evaluating the application just means substituting arguments for some of the lambda abstraction's variables, which always halts with the result a now unapplied lambda abstraction.
   
 
== See also ==
 
== See also ==

Latest revision as of 01:34, 28 June 2014

An expression is in weak head normal form (WHNF), if it is either:

  • a constructor (eventually applied to arguments) like True, Just (square 42) or (:) 1
  • a built-in function applied to too few arguments (perhaps none) like (+) 2 or sqrt.
  • or a lambda abstraction \x -> expression.

Note that the arguments do not themselves have to be fully evaluated for an expression to be in weak head normal form; thus, while (square 42) can be reduced to (42 * 42), which can itself be reduced to a normal form of 1764, Just (square 42) is WHNF without further evaluation. Similarly, (+) (2 * 3 * 4) is WHNF, even though (2 * 3 * 4) could be reduced to the normal form 24.

An exception is the case of a fully applied constructor for a data type with some fields declared as strict; the arguments for these fields then also need to be in WHNF.

The above definition might seem to treat built-in functions differently from functions defined via lambda abstraction. However, the distinction does not matter to semantics. If a lambda abstraction is applied to "too few arguments", then evaluating the application just means substituting arguments for some of the lambda abstraction's variables, which always halts with the result a now unapplied lambda abstraction.

[edit] See also