Proposal: Add Eq instance for Control.Exception.ErrorCall

Henning Thielemann lemming at
Tue Nov 27 12:12:20 CET 2012

On Tue, 27 Nov 2012, Simon Hengel wrote:

> I propose to add an Eq instance for ErrorCall.  The main motivation is
> to make it more convenient to construct predicates that select specific
> exceptions.
> My current use case is testing for expected exceptions.  In Hspec[1] we
> use predicates for that, e.g.:
>  evaluate (head []) `shouldThrow` (== ErrorCall "Prelude.head: empty list")

If this is an actual use case, then there is something very wrong. 
Handling non-empty lists can be done cleanly using various non-empty list 
types. Or you avoid 'head' by using 'case' or 'viewL'. If instead you plan 
to catch something then you should use Either, ExceptionalT, ErrorT or IO 
exceptions. I am afraid that an Eq instance for ErrorCall promotes the 
abuse of 'error'.

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