Question

I have these statements:

data SL a = SR (Integer -> (a, Integer))
    deriving(Show)

instance Monad SL where
    return k = SR (\st -> (k, st))

xx::SL Integer
xx = return 4

Then I do:

let SR f = xx

Now I have:

xx :: SL Integer
f :: Integer -> (Integer, Integer)

but I cannot understand why. Maybe I’m missing the syntactic meaning of let DATACONSTRUCTOR ...

Can you help?

Answer 1

let SR f = xx means that SR f should be equal to xx. So,

SR f = xx                   -- let
     = return 4             -- def. xx
     = SR (\st -> (4, st))  -- def. return

and thus

f = \st -> (4, st)  -- remove SR on both sides

which, in this context, is of type Integer -> (Integer, Integer) because of SR :: a -> Integer -> (a, Integer) and xx :: SL Integer.