Template Haskell is an experimental language extension to the programming language Haskell

Haskell (programming language)

Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...

 implemented in the Glasgow Haskell

Glasgow Haskell Compiler

The Glorious Glasgow Haskell Compilation System, more commonly known as the Glasgow Haskell Compiler or GHC, is an open source native code compiler for the functional programming language Haskell. The lead developers are Simon Peyton Jones and Simon Marlow...

 Compiler

Compiler

A compiler is a computer program that transforms source code written in a programming language into another computer language...

 (version 6 and later). In early incarnations it was also known as Template Meta-Haskell.

It allows compile-time metaprogramming

Metaprogramming

Metaprogramming is the writing of computer programs that write or manipulate other programs as their data, or that do part of the work at compile time that would otherwise be done at runtime...

 and generative programming by means of manipulating abstract syntax tree

Abstract syntax tree

In computer science, an abstract syntax tree , or just syntax tree, is a tree representation of the abstract syntactic structure of source code written in a programming language. Each node of the tree denotes a construct occurring in the source code. The syntax is 'abstract' in the sense that it...

s and 'splicing' results back into a program. The abstract syntax is represented using ordinary Haskell data type

Data type

In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...

s and the manipulations are performed using ordinary Haskell function

Function (mathematics)

In mathematics, a function associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output. A function assigns exactly one output to each input. The argument and the value may be real numbers, but they can...

s.

'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

Expression (programming)

An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...

, perform some transformation on the expression and splice the result back into the program. It could be written as:

foo = $( transform [| bar |] )

External links

• Template Haskell Wiki Page