Question

Hi I can’t figure this out myself and i also couldn’t find an example online

I’m trying to use a maybe or a guard and the examples i found have just two variables,
when i edit or follow the examples with more then two i get an error heres what i am trying to do

--maybe example
    Fun :: Double -> Double -> Double -> Maybe Double
    Fun a b c
        | a >= -1.0 || a <= 1.0 || b >= -1.0 || b <=1.0 || c /=0 || c >= -1.0 = Nothing
        | otherwise = Just max(  ((c^ (-c)) +   (a^(-c)-1.0)  ^   (a+ (-1.0/a)))     0.0)

error with gchi

 The function `Just' is applied to two arguments,
    but its type `a0 -> Maybe a0' has only one

while hlint gives

No suggestions

trying to use a guard instead i get a different error

--guard example
    Fun :: Double -> Double -> Double -> Maybe Double
    Fun a b c
        | a >= -1.0 || a <= 1.0 || b >= -1.0 || b <=1.0 || c /=0 || c >= -1.0 = error "Value out of range "
        | otherwise = max(  ((c^ (-c)) +   (a^(-c)-1.0)  ^   (a+ (-1.0/a)))     0.0)

and heres ghc and hlints errors

  test.hs:10:1:
        Invalid type signature: Fun :: Double
                                       -> Double -> Double -> Maybe Double
        Should be of form <variable> :: <type>
    $ hlint test.hs
 Error message:
  Left-hand side of type signature is not a variable: Fun
Code:
    Fun :: Double -> Double -> Double -> Maybe Double
  > Fun a b c

Answer 1

You must write functions with a lower-case letter. This means writing this:

fun :: Double -> Double -> Double -> Maybe Double
fun a b c ...

Functions can never start with upper-case letters, since upper-case identifiers are reserved for modules (Data.List for example) and type constructors and data constructors (like Int or Maybe or Just) and some other things.

You also have an issue with the application of the Just constructor. If you write this:

Just max(something)

… it means the same thing as:

Just max something

i.e. you are giving Just two arguments. You need to fix this by adjusting your parens:

Just (max something)