CODEWITHSUNDEEP
haskell
Saczew
Saczew

Reputation: 323

Rigid type variable in Haskell

I don't really get what is wrong with the following code.

data TypeA = TypeA

class MyClass a where
  myClassFunction :: a -> String

instance MyClass TypeA where
  myClassFunction TypeA = "TypeA"

bar :: (MyClass a) => String -> a
bar "TypeA" = TypeA

I get following error: 

   Couldn't match expected type ‘a’ with actual type ‘TypeA’
      ‘a’ is a rigid type variable bound by
          the type signature for bar :: MyClass a => String -> a
          at test.hs:9:8
    Relevant bindings include
      bar :: String -> a (bound at test.hs:10:1)
    In the expression: TypeA
    In an equation for ‘bar’: bar "TypeA" = TypeA
Failed, modules loaded: none.

I'm afraid that I'm missing something crucial about Haskell type system.

Upvotes: 10

Views: 6903

Answers (3)

Tarek Soliman
Tarek Soliman

Reputation: 71

a should be any type, and yet you return a specific type. One way around it is to use the extension existentialQuantification. Or you extend your class as the following:

data TypeA = TypeA

class MyClass a where
  myClassFunction :: a -> String
  foo :: String -> a

instance MyClass TypeA where
  myClassFunction TypeA = "TypeA"
  foo "TypeA" = TypeA
  foo _ = error "unknown"

bar :: (MyClass a) => String -> a
bar = foo

So when you call the bar function:

bar "TypeA" :: TypeA

Upvotes: 0

Bartek Banachewicz
Bartek Banachewicz

Reputation: 39370

(MyClass a) => String -> a

Means that the function can return any a type that's asked from it. Your implementation returns one particular type that satisfies that constraint. This is more obvious with the explicit signature:

bar :: forall a. (MyClass a) => String -> a

Read plainly, that's for every type a that satisfies MyClass, this function will take a string and return a value of that type. Your version would assume exists a instead.

Upvotes: 11

chepner
chepner

Reputation: 531165

The function type MyClass a => String -> a indicates a function that can return a value of any type (with an instance of MyClass) of the caller's choosing. There is no apparent way, for example, to return a value of type Int (again, assuming MyClass Int is defined) given the value TypeA

Upvotes: 6

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