On 25 oct. 2014, at 18:25, Adam Gundry
Hi Alexey,
On 25/10/14 16:42, Alexey Muranov wrote:
i am trying to understand how typeclasses work. I know that they can be used as follows (from the manual):
data Foo = Foo {x :: Integer, str :: String}
instance Eq Foo where (Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2)
I am wondering, why is the following seemingly unambiguous syntax not allowed too?
data Foo = Foo { x :: Integer, str :: String }
instance Eq Foo
(Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2)
This case is obviously unambiguous, but what is the general rule? Is the presence of the Foo data constructor important? What if I wrote the following, which would be a perfectly good instance method:
foo1 == foo2 = (x foo1 == x foo2) && (str foo1 == str foo2)
In general, should the compiler be doing type inference to determine which instance a declaration belongs to? In principle, the language could permit instance methods to be detached from the instances themselves, but I don't think it should.
Maybe to specify the types without type inference, something like this could be used: (foo1 :: Foo) == (foo2 :: Foo) = (x foo1 == x foo2) && (str foo1 == str foo2) Alexey. P.S. Maybe all i say is nonsense, I am not very familiar with the theory of types. I plan to read the Hindley's paper when i have time.