Allow expressions in patterns

I've outlined a proposal for extending pattern synonyms to depend on terms (PatternFamilies), the name is less than ideal but I'll keep the page under that name for now. The proposal's introduction has been rewritten.

The simplest use case are patterns that matches only when a set contains an element (IsMember) or when a set does not contain an element (NotMember):

[[Image(wiki:PatternFamilies:member.png)]]

    hasKeys :: Set Item -> IO ()
    hasKeys (IsMember  "keys") =             leaveHouse
    hasKeys (NotMember "keys") = findKeys >> leaveHouse

or a pattern that matches a map if the key exists:

[[Image(wiki:PatternFamilies:lookup.png)]]

used like this:

    age :: Map Person Age -> String
    age (Lookup "Alice" 60)  = "Alice is 60 years old!"
    age (Lookup "Alice" age) = "Alice is " ++ show age ++ "."
    age (Lookup "Bob" _)     = "No info on Alice but we know how old Bob is."
    age _                    = "..."

Further details and examples can be found here: PatternFamilies, I discussed it with several people ICFP 2014. This would not require a new extension but would extend PatternSynonyms. This feature can be quite useful, especially when working with deeply nested structures (ASTs, JSON, XML, ...).

If people are fine with it I will implement it.

Trac metadata

Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture