I would like to write a function that turns a function (a → b) into a function (s → t) using Lens stab (edit: I realized that this function already exists with Setter s t a band is called overor %~, but it does not answer the question below using lens to get the value) . It seems simple, but I get a confusing type error. To make an even more minimal example, consider the following function, which simply returns the value extracted from the second argument by the lens:
f :: Lens s t a b -> s -> a
f l s = s ^. l
This does not compile. There are 2 errors, as in the second argument ^. (namely, l):
- Failed to match type 't' with 's'
- Failed to match type 'a' with 'b'
However, the following compilations:
f :: Getter s a -> s -> a
f l s = s ^. l
, Getter s = t, a = b. Lens s t a b a s?