1: The problem in the example
f :: State s a
f = State $ \x -> y
where y = ... x ...
is the parameter x
. Things in the where
clause can refer only to the parameters of the function f
(there are none) and things in outer scopes.
2: To use a where
in the first example, you can introduce a second named function
that takes the x
as a parameter, like this:
f = State f'
f' x = y
where y = ... x ...
or like this:
f = State f'
where
f' x = y
where y = ... x ...
3: Here is a complete example without the ...
‘s:
module StateExample where
data State a s = State (s -> (a, s))
f1 :: State Int (Int, Int)
f1 = State $ \state@(a, b) ->
let
hypot = a^2 + b^2
result = (hypot, state)
in result
f2 :: State Int (Int, Int)
f2 = State f
where
f state@(a, b) = result
where
hypot = a^2 + b^2
result = (hypot, state)
4: When to use let
or where
is a matter of taste. I use let
to emphasize a computation (by moving it to the front) and where
to emphasize the program flow (by moving the computation to the back).