[Haskell-beginners] ignature for a list of a set length

[edgar klerks]

I think it is simpler to use the Maybe type for this. If you only  search
for ternary relations, you can write a function, which only converts a list
to a 3-tuple if it is of length three and otherwise it returns nothing. Then
with catMaybes :: [Maybe a] -> [a], you can convert it back to a list with
only 3-tuples.

import Data.Maybe

getLengthThree = catMaybes . fmap toTup
    where toTup (x:y:z:[]) = Just (x,y,z)
          toTup _ = Nothing
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