Error -> Except migration

[Ganesh Sittampalam]

Hi,

In recent versions of transformers, 'ErrorT' has been deprecated in
favour of 'ExceptT':

https://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-Trans-Error.html

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've spent a while searching for past discussion of the change, but
haven't found anything, so please do point me at anything relevant if
this has already been discussed.

Cheers,

Ganesh