3 Mar
2015
3 Mar
'15
2:44 a.m.
On Tue, 3 Mar 2015, Ganesh Sittampalam wrote:
https://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-...
This doesn't seem like a trivial like-for-like replacement since the implementation of 'fail' is different - in ErrorT it produces an exception within the ErrorT itself, and in ExceptT it just calls fail in the underlying monad.
Is there any guidance or simple trick to find if a program is relying on the old behaviour? Otherwise migrating could be rather painful/dangerous, particularly as tests often don't cover exceptional cases well.
I think you should never call 'fail' in ExceptT, only throwE or mzero. That is, all pattern matches on the left hand side of '<-' must succeed.