Variables in Haskell

Question

Why does the following Haskell script not work as expected?

find :: Eq a => a -> [(a,b)] -> [b]
find k t = [v | (k,v) <- t]

Giv

Answer 1

The pattern match (k,v) <- t in the first example creates two new local variables v and k that are populated with the contents of the tuple t. The pattern match doesn't compare the contents of t against the already existing variable k, it creates a new variable k (which hides the outer one).

Generally there is never any "variable substitution" happening in a pattern, any variable names in a pattern always create new local variables.