Simplify Haskell Type Signatures
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further ghci tells us
add :: Num a => (a -> a -> a) -> a -> a -> a
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When are type signatures necessary in Haskell?
Date : March 29 2020, 07:55 AM
it should still fix some issue Monomorphism restriction If you have MonomorphismRestriction enabled, then sometimes you will need to add a type signature to get the most general type: {-# LANGUAGE MonomorphismRestriction #-}
-- myPrint :: Show a => a -> IO ()
myPrint = print
main = do
myPrint ()
myPrint "hello"
myValue :: Read a => a
myValue = read "0"
myTuple :: Read a => (a, String)
myTuple = (myValue, "hello")
myString = snd myTuple
myString = snd (myTuple :: ((), String))
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Haskell type signatures: Question on type signature of lastButOne Program
Date : March 29 2020, 07:55 AM
To fix the issue you can do I'm reading through Real World Haskell and one of the exercises is to construct a function lastButOne, which simply returns the second to last element of a list. , You can acomplish that with pattern matching like: lastButOne :: [a] -> a
lastButOne [] = error "not elements"
lastButOne [x] = x
lastButOne [x,_] = x
lastButOne (x:xs) = lastButOne xs
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Type Signatures in Haskell
Date : March 29 2020, 07:55 AM
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Haskell Type Signatures
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , Those are constraints to a polymorphic type. Haskell, despite being statically typed, makes it very easy to write function that can work with different types, through a system called parametric polymorphism. I'll first give all your functions concrete (monomorphic) signatures instead: mult :: [Integer] -> Integer
mult = foldl' (*) 1 -- foldl' is better for performance/memory than foldr
posList :: [Integer] -> [Integer]
posList = filter (>0)
trueList :: [Bool] -> Bool
trueList = foldl' (&&) True
evenList :: [Integer] -> Bool
evenList x = foldl' (+) 2 x `mod` 2 == 0
-- `if True then True else False` is tautological
mult :: ∀ a . [a] -> a
mult :: [a] -> a
mult :: Num a => [a] -> a
posList :: [a] -> [a]
posList :: (Num a, Ord a) => [a] -> [a]
evenList :: Integral a => [a] -> Bool
evenList :: (Integral a, Foldable t) => t a -> Bool
mult :: (Num a, Foldable t) => t a -> a
trueList :: Foldable t => t Bool -> Bool
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