Use matched value in case construct in Haskell

Question

I'm pattern matching on the data constructor of a record, and I have the following chunk of code:

colorFor shape = 
  case material shape of
    ColorMaterial -> material shape
    -- etc.

The problem is this: material is a nontrivial method, and I'd like to not recompute it in the case statement. I know I could do something like:

colorFor shape = 
  let m = material shape
  in case m of
    ColorMaterial -> m

or

colorFor shape = 
  case material shape of
    ColorMaterial r g b -> ColorMaterial r g b

But I couldn't help but think there must be some way to retrieve the matched value in a pattern match. This also comes up for function definitions, where I'd like to match on the data constructor for some argument without completely unpacking it.

FYI: I'm new to Haskell, and if there are better ways of doing what I'm doing I'm very open to suggestion. Any help greatly appreciated!

Answer 1

You are looking for "as patterns":

data SomeDataType = ColorMaterial Int Int Int
                  | BlandMaterial

colorFor shape = 
  case material shape of
    res@(ColorMaterial _ _ _) -> res
    -- etc.