I'm changing some Haskell code from using lists to sets. I understand everything required, I think, but I'm not sure how to pattern match on sets. Lists have this nice literal syntax that seems hard to emulate with the Set constructor. For example, I might have some code like this:
foo [] = []
foo x = other_thing
How can I write this code so it uses Sets instead of lists?
Well, you can't.
Set is an abstract data type[0] that deliberately hides its internal representation, primarily to maintain invariants of the data structure that can't be statically-enforced by the type system (specifically, the standard library Data.Set.Set is a binary search tree).
Losing the ability to pattern match on an abstract data type is an unpleasant bit of collateral damage, but oh well. Your options are roughly:
null, as in trinithis's answer.Set to a list. Most of the time this is silly but if you want to iterate through the set anyway, it works well enough.ViewPatterns extension, which provides syntactic sugar for using accessor functions where a pattern match would normally go.Set, treat it like a set, and work with it as a whole for mapping, filtering, etc. Not always possible, but can lead to cleaner code with fewer explicit conditionals/iterations.View patterns would let you write something that looks like this:
foo (setView -> EmptySet) = []
foo (setView -> NonEmpty set) = other_thing
...where setView is a function you write. Not really much of a gain here, but can be nice for more complex pseudo-patterns
For avoiding explicit checks, besides well-known set operations such as union and intersection, consider making use of the filter, partition, map, and fold functions in Data.Set.
[0]: See this paper (warning: PDF) for the definition of the term as I'm using it.