In lazy functional languages, a refutable pattern is one which
may fail to match. An expression being matched against a
refutable pattern is first evaluated to head normal form
(which may fail to terminate) and then the top-level
constructor of the result is compared with that of the
pattern. If they are the same then any arguments are matched
against the pattern's arguments otherwise the match fails.
An irrefutable pattern is one which always matches. An
attempt to evaluate any variable in the pattern forces the
pattern to be matched as though it were refutable which may
fail to match (resulting in an error) or fail to terminate.
Patterns in Haskell are normally refutable but may be made
irrefutable by prefixing them with a tilde (~). For example,
Patterns in Miranda are refutable, except for tuples which are
irrefutable. Thus
g [x] = 2
g undefined ==> undefined
f (x,y) = 1
f undefined ==> 1
Pattern bindings in local definitions are irrefutable in both
languages:
h = 1 where [x] = undefined ==> 1
Irrefutable patterns can be used to simulate unlifted products
because they effectively ignore the top-level constructor of
the expression being matched and consider only its components.
