Template Haskell
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Template Haskell is an experimental language extension to the
Haskell programming language implemented in the Glasgow Haskell Compiler (version 6 and later).[1]
In early incarnations it was also known as Template Meta-Haskell.
It allows compile-time
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+ |]
Example
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 |] )
References
- S2CID 6096655.
External links
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