What do you do when you are dealing with what seems like a bizarre compiler bug, with the compiler being nothing less than GHC? First, pinch yourself — check; then try again, 3 times to be sure — check; clear out 'dist/' and any temporary build files — check; sleep on it — check.
And it's still happening.
I'm trying to use HStringTemplate for my personal blog software, in particular the renderf function. I was getting tricky compilation errors, and in the course of messing around I found the following:
GHC cannot compile a certain function, call it func1 for now, which uses renderf. But it compiles and works just fine if another function func2 (which doesn't use renderf, but does use a related HStringTemplate function render) is present in the module, even though func2 is not used anywhere in the project. Changing some of the details of what func2 does causes compilation to fail again, though other details can be changed.
That has to be impossible, right? Am I losing my mind?
Ideally I'd create a nice simple test case, but that might take hours, and changing small things about the voodoo function func2 seems to destroy its magical properties, and I'm suspecting the problem is in me. So I'll just post all my code. The bad news is there are lots of dependencies. The good news is I have used cabal, so the following instructions should suffice if you have cabal installed.
Download and install 'ella' (CGI web framework I'm writing) and dependencies:
hg clone -r 30aa625a3f51 http://bitbucket.org/spookylukey/ella/ cd ella/ cabal configure --user && cabal build && cabal install --user cd ..
(You can download a tarball if you don't have mercurial)
Download and build the blog software:
hg clone -r e1015168e56b http://bitbucket.org/spookylukey/haskellblog/ cd haskellblog/ cabal configure --user && cabal build
(Again, download the tarball if you don't have mercurial)
The build should succeed. Now, the voodoo function is at the end of src/Blog/Views.hs. Comment it out:
perl -pi -e 's/this_is_not_used/-- this_is_not_used/' src/Blog/Views.hs
Build again:
cabal build
Result - this compilation error.
I don't know whether that compilation error is correct or not, but either way, it seems crazy that it could depend on the existence and implementation of a completely unused function.
For reference, I'm using GHC 6.10.1.
Any ideas?
Comments §
You can get rid of the problem by giving get_template a type signature. get_template :: String -> IO (StringTemplate ByteString). With the monomorphism restriction, this_is_not_used gives get_template this concrete type.
The problem is that the StringTemplate's Stringable type parameter isn't specified and the type system doesn't know that (([Char], [(Blog.Category.Category, [Char])]) -> ([Char], Bool) -> ByteString) is never going to be Stringable.
Blog/Views.hs:57:22:
Ambiguous type variable `a' in the constraint:
`Stringable a'
arising from a use of `getStringTemplate' at Blog/Views.hs:57:22-53
Possible cause: the monomorphism restriction applied to the following:
get_template :: String -> IO (StringTemplate a)
(bound at Blog/Views.hs:55:0)
get_templates :: IO (STGroup a) (bound at Blog/Views.hs:50:0)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
Thanks again for the time and effort spent in debugging this for me :-)