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7. Other Haskell utility programs

This section describes other program(s) which we distribute, that help with the Great Haskell Programming Task.

7.1 Makefile dependencies in Haskell: using mkdependHS

You run mkdependHS like this:

  mkdependHS [mkdependHS options] [-- GHC options --] srcfile1 [srcfile2 ...]
  ghc -M [mkdependHS options(prefix with -optdep)] [ GHC options ] srcfile1 [srcfile2 ...]
To see mkdependHS's command-line flags, give it a duff flag, e.g., mkdependHS -help.

In general, if module A contains the line

        import B ...blah...
then mkdependHS will generate a dependency line of the form:
        A.o : B.hi
If module A contains the line
        import {-# SOURCE #-} B ...blah...
then mkdependHS will generate a dependency line of the form:
        A.o : B.hi-boot
(See Section Interface files for details of interface files.) If A imports multiple modules, then there will be multiple lines with A.o as the target.

By default, mkdependHS generates all the dependencies, and then concatenates them onto the end of makefile (or Makefile if makefile doesn't exist) bracketed by the lines "# DO NOT DELETE: Beginning of Haskell dependencies" and "# DO NOT DELETE: End of Haskell dependencies". If these lines already exist in the makefile, mkdependHS deletes the old dependencies first.

mkdependHS takes GHC options between -- brackets. It understands the following ones. Any options between -- brackets that it doesn't understand are simply ignored; this way you can feed your Makefile's standard GHC options to mkdependHS un-filtered.


Run the C pre-processor over the input files. The default is not to.


A cpp #define; usual meaning.


Add <dirs> (colon-separated) to list of directories to search for "import"ed modules.


Add <dir> to list of directories to search for .h files (i.e., usual meaning).

-syslib <blah>

This program uses this GHC system library; take appropriate action (e.g., recognise when they are "import"ing a module from that library).

Here are the mkdependHS-specific options (not between --'s):


Be verbose.

-v -v

Be very verbose.


Turn off warnings about interface file shadowing.

-f blah

Use blah as the makefile, rather than makefile or Makefile. If blah doesn't exist, mkdependHS creates it. We often use -f .depend to put the dependencies in .depend and then include the file .depend into Makefilpe.

-o <osuf>

Use .<osuf> as the "target file" suffix ( default: o). Multiple -o flags are permitted (GHC2.05 onwards). Thus "-o hc -o o" will generate dependencies for .hc and .o files.

-s <suf>

Make extra dependencies that declare that files with suffix .<suf>_<osuf> depend on interface files with suffix .<suf>_hi, or (for {-# SOURCE #-} imports) on .hi-boot. Multiple -s flags are permitted. For example, "-o hc -s a -s b" will make dependencies for .hc on .hi, .a_hc on .a_hi, and .b_hc on .b_hi. (Useful in conjunction with NoFib "ways".)


Regard <file> as "stable"; i.e., exclude it from having dependencies on it.


same as --exclude-module


Regard the colon-separated list of directories <dirs> as containing stable, don't generate any dependencies on modules therein.


same as --exclude-directory.


Regard <file> as not "stable"; i.e., generate dependencies on it (if any). This option is normally used in conjunction with the --exclude-directory option.


Regard prelude libraries as unstable, i.e., generate dependencies on the prelude modules used (including Prelude). This option is normally only used by the various system libraries. If a -syslib option is used, dependencies will also be generated on the library's interfaces.

7.2 Emacs `TAGS' for Haskell: hstags

`Tags' is a facility for indexing the definitions of programming-language things in a multi-file program, and then using that index to jump around among these definitions.

Rather than scratch your head, saying ``Now where did we define `foo'?'', you just do (in Emacs) M-. foo RET, and You're There! Some people go wild over this stuff...

GHC comes with a program hstags, which build Emacs-able TAGS files. The invocation syntax is:

hstags [GHC-options] file [files...]

The best thing is just to feed it your GHC command-line flags. A good Makefile entry might be:

        $(RM) TAGS
        hstags $(GHC_FLAGS) *.lhs

The only flags of its own are: -v to be verbose; -a to **APPEND** to the TAGS file, rather than write to it.

Shortcomings: (1) Instance declarations don't get into the TAGS file (but the definitions inside them do); as instances aren't named, this is probably just as well. (2) Data-constructor definitions don't get in. Go for the corresponding type constructor instead.

(Actually, GHC also comes with etags [for C], and perltags [for You Know What]. And---I cannot tell a lie---there is Denis Howe's fptags [for Haskell, etc.] in the ghc/CONTRIB section...)

7.3 ``Yacc for Haskell'': happy

Andy Gill and Simon Marlow have written a parser-generator for Haskell, called happy. Happy is to Haskell what Yacc is to C.

You can get happy by FTP from in pub/haskell/happy, the file happy-0.8.tar.gz.

Happy is at its shining best when compiled by GHC.

7.4 Pretty-printing Haskell: pphs

Andrew Preece has written pphs, a utility to pretty-print Haskell code in LaTeX documents. Keywords in bolds, variables in italics---that sort of thing. It is good at lining up program clauses and equals signs, things that are very tiresome to do by hand.

The code is distributed with GHC in ghc/CONTRIB/pphs.

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