OrPatterns

Quasiquoter for or patterns separated by " | ", like other languages (ML family).

Example:

>>> :set -XQuasiQuotes -XViewPatterns -w

>>> :{
>>> let f :: Either (a,b) (b,a) -> (a,b)
>>> f [o| Left (x,y) | Right (y,x) |] = (x,y)
>>> :}

>>> map f [Left ('a',0), Right (2, 'b')]
[('a',0),('b',2)]

A more confusing example (to show that the string | is interpreted correctly by the parser):

>>> :{
>>> let g [o| " | " | "|||" | "  |  " |] = True
>>> g _ = False
>>> :}

>>> map g [ "|", " | " , "|||" , "  |  "]
[False,True,True,True]

f is desugared to something like:

 f ( (\v -> case v of
             Left (x,y) -> Just (x,y)
             Right (y,x) -> Just (x,y)
             _ -> Nothing
      ) -> Just (x,y) ) = (x,y)

So failing to match will pass on to the next equation.

Limitations include:

• compilation could probably be more efficient
• incorrect patterns, such as those binding the wrong variables or the wrong number of variables could be reported better
• generated code can have overlapped patterns, and so -Wall doesn't help the above example. Duplicating or using ghc's check for this should be done, in which case the the desugared code should look like f2.

 f2 ( (\v -> case v of
         Left (x,y) -> (x,y)
         Right (y,x) -> (x,y))
        -> a) = a

Documentation

o :: QuasiQuoter

or pattern quasiquote. See above.