# Eq your errors

December 15, 2015

When building out error types in Haskell, we must derive Eq or the compiler can’t pattern-match on them.

One instance of this is for using case expressions where the constructor is the condition. Thus, if the error’s constructor is the condition, we must have a way to compare constructors. We could, technically, build our own custom Eq definition, but for something as simple as errors, why would you want to?