Chapter 6. Invoking Alex

The command line syntax for Alex is entirely standard:

$ alex { option } file.x  { option }

Alex expects a single file.x to be named on the command line. By default, Alex will create file.hs containing the Haskell source for the lexer.

The options that Alex accepts are listed below:

-o file, --outfile=file

Specifies the filename in which the output is to be placed. By default, this is the name of the input file with the .x suffix replaced by .hs.

-i [file], --info [=file]

Produces a human-readable rendition of the state machine (DFA) that Alex derives from the lexer, in file (default: where the input file is file.x).

The format of the info file is currently a bit basic, and not particularly informative.

-t [dir], --template=dir

Look in dir for template files.

-g, --ghc

Causes Alex to produce a lexer which is optimised for compiling with GHC. The lexer will be significantly more efficient, both in terms of the size of the compiled lexer and its runtime.

-d, --debug

Causes Alex to produce a lexer which will output debugging messsages as it runs.

-l, --latin1

Disables the use of UTF-8 encoding in the generated lexer. This has two consequences:

  • The Alex source file is still assumed to be UTF-8 encoded, but any Unicode characters outside the range 0-255 are mapped to Latin-1 characters by taking the code point modulo 256.

  • The built-in macros $printable and '.' range over the Latin-1 character set, not the Unicode character set.

Note that this currently does not disable the UTF-8 encoding that happens in the "basic" wrappers, so --latin1 does not make sense in conjunction with these wrappers (not that you would want to do that, anyway).

-?, --help

Display help and exit.

-V, --version

Output version information and exit. Note that for legacy reasons -v is supported, too, but the use of it is deprecated. -v will be used for verbose mode when it is actually implemented.