Question

I’d like to write a function that takes both

• a value constructor for a certain algebraic data type, and
• an actual value of that same type,

and determines whether the given value is “made from” the given constructor. Pattern matching seems like a natural fit for this, but the pattern to match against would have to be a function parameter instead of a hard-coded constructor name.

The code below is what I’ve tried, but GHC reports a parse error on the line indicated.

Is there a way to accomplish this?

data FooBar = Foo Int | Bar String

-- Imagine that these are useful functions.
processInt :: Int -> String
processInt = show
processString :: String -> String
processString = id

-- This should take one of the above functions and adapt it to operate on
-- FooBar values of compatible "type".  Values that match the given FooBar
-- constructor should be "unwrapped" and passed to the given function.
typeCheck :: (a -> FooBar) -> (a -> String) -> (FooBar -> Maybe String)
typeCheck constructor func fooBar = case fooBar of
  (constructor x) -> Just (func x)  -- GHC says "Parse error in pattern: constructor"
  _ -> Nothing

-- Define processing functions that operate on FooBars.
processFoo :: FooBar -> Maybe String
processFoo = typeCheck Foo processInt
processBar :: FooBar -> Maybe String
processBar = typeCheck Bar processString

Answer 1

Interesting idea. I wonder what you’re trying to do, since this is quite an unusual pattern matching problem.

You can certainly do it if you can:

• enumerate the constructors of the type
• have equality on the type’s elements

Like so (I break out the application of f part, since that is orthogonal):

wasBuilt :: Eq t => (t -> Either t t)   -- ^ the constructor
                 -> Either t t          -- ^ a value
                 -> Maybe t             -- ^ the transformed result

wasBuilt k v = case v of
    Left  x | v == k x    -> Just x
    Right x | v == k x    -> Just x
    _                     -> Nothing

But there’s a lot of boilerplate. This problem screams “generics” at me. Try a different approach, and reflect the constructor to data, then match generically on that data, perhaps. This will allow you to treat the constructors as values, instead of functions.

---

Here’s roughly what I was thinking, but note, this is an advanced technique. Explicit pattern matching on a regular AST is much, much more idiomatic:

import Generics.SYB

-- only works for unary constructors
sameConstructor :: (Data a, Data b) => (a -> b) -> b -> Bool
sameConstructor k v = toConstr v == toConstr (k undefined)

> sameConstructor (Left :: Char -> Either Char Char) (Right 'x')
False

> sameConstructor (Left :: Char -> Either Char Char) (Left 'x')
True

> sameConstructor (:[]) "haskell"
True