[Haskell-cafe] problem with happy and type identity

Erik Hesselink hesselink at gmail.com
Wed Feb 5 19:34:40 UTC 2014

To be able to share code between your library and your tests, you should either

* Have a separate directory for the library sources and the test
sources. For example, put Ast.hs and Parser.y in 'src' and put
'Hs-source-dirs: src' in the library section. Put RunTests.hs in
'tests', and put 'Hs-source-dirs: tests' in the test-suite section.
This is the preferred way.
* Alternatively, don't specify 'sample' as a build-depends in the
test-suite. Note that this will mean that Ast and Parser will get
compiled twice.

Right now, cabal somehow mixes types from the files compiled as part
of the test suite with types imported from the library.



On Sun, Feb 2, 2014 at 10:32 PM, Richard Cobbe <cobbe at ccs.neu.edu> wrote:
> I'm working on a small project that involves an Alex scanner and a Happy
> parser, and I'm getting an error from the type-checker that I do not
> understand.  Can anyone help shed some light on what's going on?
> I'm running Haskell Platform 2013.2.0.0, on MacOS 10.8.5 with XCode 4.6.3.
> I've reduced the problem to a very small example, which I've attached as a
> tar file.  It's a cabal package, and it contains a library with some test
> cases.
> If I run "cabal configure && cabal build" then the library builds with no
> problems whatsoever.
> But if I run
>     cabal clean && cabal configure --enable-tests && cabal build
> then I get the following error message:
>     RunTests.hs:16:27:
>         Couldn't match expected type `sample-0.4:Ast.Entry'
>                     with actual type `Entry'
>         In the return type of a call of `Entry'
>         In the second argument of `(~?=)', namely `Entry "mumble"'
>         In the expression: parse "entry" ~?= Entry "mumble"
> And this doesn't make any sense to me, because the two types are supposed
> to be the same.
> If I take Happy and Alex out of the picture by replacing the Parser module
> with a hand-written parser, the error goes away.  This is hardly feasible
> in the actual project, of course :-) so I'm hoping someone can shed some
> light on what I'm doing wrong here.
> Thanks!
> Richard
> (I apologize for the attachment, by the way, but since filesystem layout is
> potentially important here, and since it's only slightly larger than 1K, I
> figured it was the best way.  If there's a better way to do this in the
> future (which doesn't assume I can throw it on the web somewhere), then I'd
> love to hear about it.)
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