How to ignore a polymorph readable type, while reading?

Question

I have a problem similar to:

data Foo a = Foo { myInt :: Integer, myValue :: a } deriving Read

bar :: String -> Integer
bar = myInt . read

main = print $ bar stringWithFooOfa

i don't want to use something like read . (!!1) . words, if i don't need to.

Answer 1

Add a type annotation to read. As commenters suggested, Haskell has no way of knowing what you're trying to read since you immediately turn it into an Int. To be more clear, consider this example:

data Foo a  = Foo { myInt  :: Integer, myValue  :: a } deriving Read
data Foo2 a = Foo { myInt2 :: Integer                } deriving Read

bar :: String -> Integer
bar = myInt . read

Now there are two very different behaviors for bar possible and it's hard to know which is correct.

To tell Haskell which one you want, use an inline annotation around read:

bar :: String -> Integer
bar = myInt . (read :: String -> Foo ())

Notice that I pick an a, too. Otherwise, we'll be in the same boat as above but just with a instead of Foo.