alex

Alex is a tool for generating lexical analysers in Haskell

http://www.haskell.org/alex/

Version on this page:3.2.4
LTS Haskell 24.16:3.5.4.0
Stackage Nightly 2025-10-24:3.5.4.0
Latest on Hackage:3.5.4.0

See all snapshots alex appears in

BSD-3-Clause licensed by Chris Dornan and Simon Marlow
Maintained by Simon Marlow
This version can be pinned in stack with:alex-3.2.4@sha256:fcf1971fb513eab886072d1f705475f04fb65261a4d81361bede9d86056b474b,4054

Module documentation for 3.2.4

There are no documented modules for this package.

Alex: A Lexical Analyser Generator

Build Status

Alex is a Lex-like tool for generating Haskell scanners. For complete documentation, see the doc directory.

Alex version 2.0 has changed fairly considerably since version 1.x, and the syntax is almost completely different. For a detailed list of changes, see the release notes in the documentation.

Alex is now covered by a BSD-Style licence; see the licence file in the ‘doc’ directory for details.

The sources are in the ‘src’ directory and the documentation in the ‘doc’ directory; various examples are in the ‘examples’ subdirectory.

The source code in the ‘src’ and ‘examples’ directories is intended for a Haskell 98 compiler with hierarchical modules. It should work with GHC >= 5.04.

Build Instructions

If you just want to use Alex, you can download or install (via cabal install alex) an Alex release from Hackage; also note that distributions such as the Haskell Platform and other package manager-based distributions provide packages for Alex. Moreover, recent versions of cabal will automatically install the required version of alex based on build-tools/build-tool-depends declarations.

Read on if you want to build Alex directly from Git.

Alex is built using GHC & Cabal; so first install GHC and cabal-install-2.0 (or later).

Since Alex itself is implemented in terms of an Alex scanner, bootstrapping Alex is a bit tricky:

You need to have the build-tools alex and happy manually installed; either via your system package manager distribution, the Haskell Platform, or e.g. via (run this outside the Git repository!):

$ cabal install alex happy

which installs them into ${HOME}/.cabal/bin by default (make sure they’re in your $PATH for the next steps!).

Variant A

First you need to generate the pre-processed templates via

$ cabal new-run gen-alex-sdist

(otherwise cabal install will complain about “data/AlexTemplate: copyFile: does not exist (No such file or directory)”)

And then you can install alex simply by invoking

$ cabal install

from inside the Git folder.

Variant B

Alternatively, you can use the Makefile which automates the steps of producing a self-contained pre-bootstrapped source distribution with pre-generated lexer/scanners (and which also performs the cabal new-run gen-alex-sdist pre-preprocessing step):

$ make sdist
$ cabal install dist/alex-*.tar.gz

For convenience, there’s also a make sdist-test target which builds the source source tarball and runs the test-suite from within the source dist.

Contributing & Reporting Issues

Please report any bugs or comments at https://github.com/simonmar/alex/issues

Share and enjoy,

Chris Dornan: [email protected]

Isaac Jones: [email protected]

Simon Marlow: [email protected]

Changes

Changes in 3.2.4:

  • Remove dependency on QuickCheck
  • Change the way that bootstrapping is done: see README.md for build instructions

Changes in 3.2.3:

  • fix issue when using cpphs (#116)

Changes in 3.2.2:

  • Manage line length in generated files [GH-84]
  • Fix issue when identifier with multiple single quotes, e.g. foo'' was used
  • Allow omitting spaces around = in macro definitions
  • Include pre-generated Parser.hs and Scan.hs in the Hackage upload, to make bootstrapping easier.

Changes in 3.2.1:

  • Fix build problem with GHC; add new test tokens_scan_user.x

Changes in 3.2.0:

  • Allow the token type and productions to be overloaded, and add new directives: %token, %typeclass, %action. See “Type Signatures and Typeclasses” in the manual.
  • Some small space leak fixes

Changes in 3.1.7:

  • Add support for %encoding directive (allows to control --latin1 from inside Alex scripts)
  • Make code forward-compatible with in-progress proposals
  • Suppress more warnings

Changes in 3.1.6:

  • sdist for 3.1.5 was mis-generated, causing it to ask for Happy when building.

Changes in 3.1.5:

  • Generate less warning-laden code, and suppress other warnings.
  • Bug fixes.

Changes in 3.1.4:

  • Add Applicative/Functor instances for GHC 7.10

Changes in 3.1.3:

  • Fix for clang (XCode 5)

Changes in 3.1.2:

  • Add missing file to extra-source-files

Changes in 3.1.1:

  • Bug fixes (#24, #30, #31, #32)

Changes in 3.1.0:

  • necessary changes to work with GHC 7.8.1

Changes in 3.0 (since 2.3.5)

  • Unicode support (contributed mostly by Jean-Philippe Bernardy, with help from Alan Zimmerman).

    • An Alex lexer now takes a UTF-8 encoded byte sequence as input (see Section 5.1, “Unicode and UTF-8”. If you are using the “basic” wrapper or one of the other wrappers that takes a Haskell String as input, the string is automatically encoded into UTF-8 by Alex. If your input is a ByteString, you are responsible for ensuring that the input is UTF-8 encoded. The old 8-bit behaviour is still available via the –latin1 option.

    • Alex source files are assumed to be in UTF-8, like Haskell source files. The lexer specification can use Unicode characters and ranges.

    • alexGetChar is renamed to alexGetByte in the generated code.

    • There is a new option, --latin1, that restores the old behaviour.

  • Alex now does DFA minimization, which helps to reduce the size of the generated tables, especially for lexers that use Unicode.