Editing Template Haskell

{{Short description|Computer language extension}}
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{{More citations needed|date=October 2021}}
{{Notability|date=December 2019}}
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'''Template Haskell''' ('''Template Meta-Haskell''' for early versions) is an experimental language extension to the [[Functional programming|functional]] [[programming language]] [[Haskell]], implemented in the [[Glasgow Haskell Compiler]] (GHC) version 6 and later.<ref name="Sheard2002">{{cite book |last1=Sheard |first1=Tim |last2=Jones |first2=Simon Peyton |author2-link=Simon Peyton Jones |date=3 October 2002 |title=Proceedings of the 2002 ACM SIGPLAN workshop on Haskell |chapter=Template meta-programming for Haskell |pages=1–16 |doi=10.1145/581690.581691 |chapter-url=https://dl.acm.org/doi/10.1145/581690.581691 |publisher=Association for Computing Machinery |isbn=1581136056 |s2cid=6096655}}</ref>

It allows [[compile time]] [[metaprogramming]] and [[generative programming]] by means of manipulating [[abstract syntax tree]]s and '[[wikt:splice|splicing]]' results back into a program. The abstract syntax is represented using ordinary Haskell [[data type]]s and the manipulations are performed using ordinary Haskell [[Function (mathematics)|functions]].

'[[Quasi-quote]]' brackets <code>[|</code> and <code>|]</code> are used to get the abstract syntax tree for the enclosed expression and 'splice' brackets <code>$(</code> and <code>)</code> are used to convert from abstract syntax tree into code.

As of GHC-6.10, Template Haskell provides support for user-defined quasi-quoters, which allows users to write parsers which can generate Haskell code from an arbitrary syntax. This syntax is also enforced at compile time. For example, using a custom quasi-quoter for [[regular expressions]] could look like this:
<syntaxhighlight lang="haskell">
 digitsFollowedByLetters = [$re| \d+ \s+ |]
</syntaxhighlight>

== Example ==
A common idiom is to quasi-quote an [[expression (programming)|expression]], perform some transformation on the expression and splice the result back into the program. It could be written as:
<syntaxhighlight lang="haskell">
 result = $( transform [| input |] )
</syntaxhighlight>

== References ==
{{Reflist}}

== External links ==
* [http://www.haskell.org/haskellwiki/Template_Haskell Haskell.org Template Haskell Wiki page]

{{Haskell programming}}

[[Category:Experimental programming languages]]
[[Category:Haskell programming language family]]
[[Category:Functional languages]]
[[Category:Metaprogramming]]
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[[Category:Articles with example Haskell code]]