Alex - Lexer generator for Haskell
Alex is a lexer generator for Haskell. Please review its documentation for syntax and general use before reading this document.
One of the interesting features of Alex is that it allows the use of and
creation of wrappers which embody a significant portion of the
bullet plate code needed for a lexer. Four wrappers are provided with
Alex: basic, posn, monad and gscan.
Typically, the monad wrapper is found to be a good compromise
between complexity and usability. However, a simple modification to it,
described here, adds significantly to its usability while minimally affecting
the complexity.
The monadUser wrapper
Please note this wrapper is based on the monad wrapper included
with Alex. The code is in the public domain.
Use in the Alex source file
In Alex, wrappers are chosen by the %wrapper directive. To use
the monadUser wrapper, add the line
%wrapper "monadUser"
after the initial code block in your Alex source file.
Additionally, two items must be defined in a code block (or imported), the
Haskell type UserData and the variable userStartState
which must be of type UserData.
The template directory
The monadUser wrapper is not provided with a vanilla installation
of Alex, so must be installed in your own template directory.
This template directory must also contain the standard Alex templates.
First, create a directory alex in your home directory. Copy all
of the files from \usr\lib\alex2.0.1
to alex. Then download the file
AlexWrapper-monadUser and
store it in the alex directory.
Running Alex
When running alex, you must tell it to search the new directory for its
templates and wrappers. This is accomplished by the -t flag.
Run Alex like this:
alex -t ~/alex input.x
Last modified by
Brett GilesLast modified: Mon Aug 27 20:34:22 MDT 2007
