I am trying to check whether my second part of my tuple is 1.0 and if it is then I am storing it in the list. But I can't figure out how to implement the check.
number :: [(Integer, Double)] -> [Integer]
number lst = number' lst []
where
number' [] a = a
((mat,1.0): xs) a = number'(xs) (mat:a) -- Here I am getting my error
((_,lst): xs) a = number'(xs) a
Maybe someone has an idea.
The reason this doesn't work is because you need to write number' for every line:
number :: [(Integer, Double)] -> [Integer]
number lst = number' lst []
where
number' [] a = a
number' ((mat,1.0): xs) a = number'(xs) (mat:a)
number' ((_,lst): xs) a = number'(xs) a
But this will return the "keys" in reverse.
You can pattern match with:
number :: (Eq a, Num a) => [(a, b)] -> [a]
number ((x, 1):xs) = x : number xs
number (_:xs) = number xs
number [] = []
You can however work with list comprehension:
number :: (Eq a, Num a) => [(a, b)] -> [a]
number xs = [ x | (x, 1) <- xs ]
or with a combination of map and filter:
number :: (Eq a, Num a) => [(a, b)] -> [a]
number = map fst . filter ((1 ==) . snd)