How to examine a quoted data constructor name in Template Haskell?

Question

I\'m trying to learn some Template Haskell. As an exercise, I wrote a function that can generate things like isLeft and isRight (inspired by this q

Answer 1

While you could use reify and examine the type to determine the arity of the data constructor, it's much easier to generate arity-independent code using a record pattern:

isFoo :: Bar -> Bool
isFoo p = case p of
    (Foo {}) -> True     -- Valid no matter what the arity of Foo is
    _        -> False

This can be done by replacing conP with recP in your code.

isA connam = do
    ConE nam <- connam
    nn <- newName "p"
    lamE [varP nn] $ caseE (varE nn) [
                       match (recP nam []) ( normalB [| True |] ) [],
                       match wildP ( normalB [| False |] ) [] 
                     ]