Invoking Happy

An invocation of Happy has the following syntax:

$ happy [ options ] filename [ options ]

All the command line options are optional (!) and may occur either before or after the input file name. Options that take arguments may be given multiple times, and the last occurrence will be the value used.

There are two types of grammar files, file.y and, with the latter observing the reverse comment (or literate) convention (i.e. each code line must begin with the character >, lines which don’t begin with > are treated as comments). The examples distributed with Happy are all of the .ly form.

The flags accepted by Happy are as follows:

-o <file>; --outfile=<file>

Specifies the destination of the generated parser module. If omitted, the parser will be placed in <file>.hs, where <file> is the name of the input file with any extension removed.

-i [<file>]; --info[=<file>]

Directs Happy to produce an info file containing detailed information about the grammar, parser states, parser actions, and conflicts. Info files are vital during the debugging of grammars. The filename argument is optional. (note that there’s no space between -i and the filename in the short version), and if omitted the info file will be written to <file>.info (where <file> is the input file name with any extension removed).

-p [<file>]; --pretty[=<file>]

Directs Happy to produce a file containing a pretty-printed form of the grammar, containing only the productions, without any semantic actions or type signatures. If no file name is provided, then the file name will be computed by replacing the extension of the input file with .grammar.

-t <dir>; --template=<dir>

Instructs Happy to use this directory when looking for template files: these files contain the static code that Happy includes in every generated parser. You shouldn’t need to use this option if Happy is properly configured for your computer.

-m <name>; --magic-name=<name>

Happy prefixes all the symbols it uses internally with either happy or Happy. To use a different string, for example if the use of happy is conflicting with one of your own functions, specify the prefix using the -m option.

-s; --strict


The --strict option is experimental and may cause unpredictable results.

This option causes the right hand side of each production (the semantic value) to be evaluated eagerly at the moment the production is reduced. If the lazy behaviour is not required, then using this option will improve performance and may reduce space leaks. Note that the parser as a whole is never lazy - the whole input will always be consumed before any input is produced, regardless of the setting of the --strict flag.

-g; --ghc

Instructs Happy to generate a parser that uses GHC-specific extensions to obtain faster code.

-c; --coerce

Use GHC’s unsafeCoerce# extension to generate smaller faster parsers. Type-safety isn’t compromised.

This option may only be used in conjunction with -g.

-a; --arrays

Instructs Happy to generate a parser using an array-based shift reduce parser. When used in conjunction with -g, the arrays will be encoded as strings, resulting in faster parsers. Without -g, standard Haskell arrays will be used.

-d; --debug

Generate a parser that will print debugging information to stderr at run-time, including all the shifts, reductions, state transitions and token inputs performed by the parser.

This option can only be used in conjunction with -a.

-l; --glr

Generate a GLR parser for ambiguous grammars.

-k; --decode

Generate simple decoding code for GLR result.

-f; --filter

Filter the GLR parse forest with respect to semantic usage.

-?; --help

Print usage information on standard output then exit successfully.

-V; --version

Print version information on standard output then exit successfully. 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.