Return the number of tuples in a list whose first member is prefixed by some character

foo.hs

-- Return the number of tuples in a list whose first member is prefixed by a
foo :: Char -> [(String, a)] -> Int
foo a = foldl succIfPrefixed 0 where
  succIfPrefixed acc (x:xs, y) | x == a    = succ acc
                               | otherwise = acc

-- Or more compactly...
foo' :: Char -> [(String, a)] -> Int
foo' a = foldl (\acc (x:xs, y) -> if x == a then acc + 1 else acc) 0