[Haskell-cafe] Unexpected type error

Alexander Solla alex.solla at gmail.com
Mon Jun 13 19:34:15 CEST 2011

On Sun, Jun 12, 2011 at 7:12 PM, Antoine Latter <aslatter at gmail.com> wrote:

> Types from differing versions of the same module are specifically not
> compatible - even if they are defined identically (in this case they
> are different, though).
> You can do one of two things:
> 1. Edit the package description of the package that wants the older
> version of parsec to allow to use the new one. Also, it helps to
> slightly bump the version number of the package when you do this.
> 2. Use the older version of parsec in your software. You already have
> it installed (or else the package you want to use wouldn't be able to
> load!). If you're using ghci you can start it with the option
> '-package parsec-'.

I understand that types from different versions of a module are
incompatible.  What I don't understand is why one module is using a new
version of Parsec when it is constrained to use the "old" one (I control
both modules.  Their package descriptions contain the same line).  Is this a
cabal-dev bug?

I'll try the "-package parsec-" tip, but this is /not/ a long term
solution to my problem.
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