Glasgow Haskell Compiler

Last updated

The Glasgow Haskell Compiler
Original author(s) Kevin Hammond
Developer(s) The Glasgow Haskell Team [1]
Initial releaseDecember 1992 (1992-12) [2]
Stable release
9.0.1  OOjs UI icon edit-ltr-progressive.svg / 4 February 2021;7 days ago (4 February 2021) [3]
Repository OOjs UI icon edit-ltr-progressive.svg
Written in Haskell and C
Operating system Linux, OS X 10.7 Lion and later, iOS, Windows 2000 and later, FreeBSD, Solaris 10 and later
Platform x86, x86-64, ARM
Available inEnglish
Type Compiler
License New BSD License

The Glasgow Haskell Compiler (GHC) is an open-source native code compiler for the functional programming language Haskell. [4] It provides a cross-platform environment for the writing and testing of Haskell code and it supports numerous extensions, libraries, and optimisations that streamline the process of generating and executing code. GHC is the most commonly used Haskell compiler. [5] The lead developers are Simon Peyton Jones and Simon Marlow.



GHC originally started in 1989 as a prototype, written in LML (Lazy ML) by Kevin Hammond at the University of Glasgow. Later that year, the prototype was completely rewritten in Haskell, except for its parser, by Cordelia Hall, Will Partain, and Simon Peyton Jones. Its first beta release was on 1 April 1991 and subsequent releases added a strictness analyzer as well as language extensions such as monadic I/O, mutable arrays, unboxed data types, concurrent and parallel programming models (such as software transactional memory and data parallelism) and a profiler. [2]

Peyton Jones, as well as Marlow, later moved to Microsoft Research in Cambridge, England, where they continued to be primarily responsible for developing GHC. GHC also contains code from more than three hundred other contributors. [1] Since 2009, third-party contributions to GHC have been funded by the Industrial Haskell Group. [6]


GHC itself is written in Haskell, [7] but the runtime system for Haskell, essential to run programs, is written in C and C--.

GHC's front end—incorporating the lexer, parser and typechecker—is designed to preserve as much information about the source language as possible until after type inference is complete, toward the goal of providing clear error messages to users. [2] After type checking, the Haskell code is desugared into a typed intermediate language known as "Core" (based on System F, extended with let and case expressions). Recently, Core was extended to support generalized algebraic datatypes in its type system, and is now based on an extension to System F known as System FC. [8]

In the tradition of type-directed compilation, GHC's simplifier, or "middle end", where most of the optimizations implemented in GHC are performed, is structured as a series of source-to-source transformations on Core code. The analyses and transformations performed in this compiler stage include demand analysis (a generalization of strictness analysis), application of user-defined rewrite rules (including a set of rules included in GHC's standard libraries that performs foldr/build fusion), unfolding (called "inlining" in more traditional compilers), let-floating, an analysis that determines which function arguments can be unboxed, constructed product result analysis, specialization of overloaded functions, as well as a set of simpler local transformations such as constant folding and beta reduction. [9]

The back end of the compiler transforms Core code into an internal representation of C--, via an intermediate language STG (short for "Spineless Tagless G-machine"). [10] The C-- code can then take one of three routes: it is either printed as C code for compilation with GCC, converted directly into native machine code (the traditional "code generation" phase), or converted to LLVM virtual machine code for compilation with LLVM. In all three cases, the resultant native code is finally linked against the GHC runtime system to produce an executable.


GHC complies with the language standards, both Haskell 98 [11] and Haskell 2010. [12] It also supports many optional extensions to the Haskell standard: for example, the software transactional memory (STM) library, which allows for Composable Memory Transactions.

Extensions to Haskell

A number of extensions to Haskell have been proposed. These extensions provide features not described in the language specification, or they redefine existing constructs. As such, each extension may not be supported by all Haskell implementations. There is an ongoing effort [13] to describe extensions and select those which will be included in future versions of the language specification.

The extensions [14] supported by the Glasgow Haskell Compiler include:

Type system extensions

An expressive static type system is one of the major defining features of Haskell. Accordingly, much of the work in extending the language has been directed towards types and type classes.

The Glasgow Haskell Compiler supports an extended type system based on the theoretical System FC. [8] Major extensions to the type system include:

Extensions relating to type classes include:


Versions of GHC are available for several platforms, including Windows and most varieties of Unix (such as Linux, FreeBSD, OpenBSD, and macOS). [17] GHC has also been ported to several different processor architectures. [17]

See also

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  16. Mitchell, Neil; Fletcher, Shayne (3 May 2020). "Record Dot Syntax". ghc-proposals. GitHub . Retrieved 30 June 2020.
  17. 1 2 Platforms at