Template Haskell

This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these messages) This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Template Haskell" – news · newspapers · books · scholar · JSTOR (October 2021) (Learn how and when to remove this message) The topic of this article may not meet Wikipedia's general notability guideline. Please help to demonstrate the notability of the topic by citing reliable secondary sources that are independent of the topic and provide significant coverage of it beyond a mere trivial mention. If notability cannot be shown, the article is likely to be merged, redirected, or deleted. Find sources: "Template Haskell" – news · newspapers · books · scholar · JSTOR (December 2019) (Learn how and when to remove this message) (Learn how and when to remove this message)

Template Haskell (Template Meta-Haskell for early versions) is an experimental language extension to the functional programming language Haskell, implemented in the Glasgow Haskell Compiler (GHC) version 6 and later.^[1]

It allows compile time metaprogramming and generative programming by means of manipulating abstract syntax trees and 'splicing' results back into a program. The abstract syntax is represented using ordinary Haskell data types and the manipulations are performed using ordinary Haskell functions.

'Quasi-quote' brackets [| and |] are used to get the abstract syntax tree for the enclosed expression and 'splice' brackets $( and ) 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:

 digitsFollowedByLetters = [$re| \d+ \s+ |]

A common idiom is to quasi-quote an expression, perform some transformation on the expression and splice the result back into the program. It could be written as:

 result = $( transform [| input |] )

• Haskell.org Template Haskell Wiki page