Alex compilation fails on ambiguous type [was: Re: Alex requirement for Setup.lhs]

Philip K.F. Hölzenspies p.k.f.holzenspies at
Thu Dec 4 08:39:49 EST 2008

On Thursday 04 December 2008 14:24:32 Philip K.F. Hölzenspies wrote:
> [16 of 16] Compiling Main             ( src/Main.hs,
> dist/build/alex/alex-tmp/Main.o )
> src/Main.hs:316:25:
>     Ambiguous type variable `e' in the constraint:
>       `GHC.Exception.Exception e'
>         arising from a use of `throw' at src/Main.hs:316:25-31
>     Probable fix: add a type signature that fixes these type variable(s)
> Lines 314 through 316 in src/Main.hs are:
>     r <- Exception.catch
> 	   (unblock (thing a))
> 	   (\e -> do { after a; throw e })
> I guess the error in itself makes some sense, because throw can not be
> resolved without knowing a concrete instance of e. I just don't get why I
> get this error in a release version of alex; I would imagine other people
> do not get this error. What am I missing?

Dear Ian, all,

Please ignore my previous e-mail. It must have been a remainder from an 
earlier failed build attempt. I just did a checkout from the alex darcs 
repository, which built without a hitch.


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