How do I use putStrLn on Maybe value in Haskell Stack test?

Question

I am using the Haskell Stack to create a program. My program builds okay but I would like to write a test file so that I can run stack test on my existing program. My problem is that my function to test returns a Maybe type, which cannot be outputted using the syntax putStrLn f. I am attempting to write a function in my test file that takes a Maybe value and returns "Nothing" if it was given Nothing, or a string containing a if it was given Just a.

Here is my code so far:

printMaybe :: Show a => Maybe a -> String
printMaybe Nothing = "Nothing"
printMaybe (Just a) = show a

and main contains the line putStrLn $ printMaybe (Nothing)

My error message is:

test\Spec.hs:12:16: error: * Ambiguous type variable a0' arising from a use of printMaybe' prevents the constraint (Show a0)' from being solved. Probable fix: use a type annotation to specify what a0' should be. These potential instances exist: instance Show Ordering -- Defined in GHC.Show' instance Show Integer -- Defined in GHC.Show' instance Show a => Show (Maybe a) -- Defined in GHC.Show' ...plus 22 others ...plus 27 instances involving out-of-scope types (use -fprint-potential-instances to see them all) * In the second argument of ($)', namely `printMaybe (Nothing)' In a stmt of a 'do' block: putStrLn $ printMaybe (Nothing) In the expression: do putStrLn ("====== Tests Start ======") putStrLn $ printMaybe (Nothing) | 12 | putStrLn $ printMaybe (Nothing) | ^^^^^^^^^^^^^^^^^^^^

Any help would be much appreciated :)

As the error says, the problem is not with your printMaybe, it is with printMaybe Nothing in your do block, since then it is unclear what the a type is. — Willem Van Onsem

Willem Van Onsem Willem Van Onsem · Accepted Answer · 2020-11-07T15:37:00

As the error says, the problem is not with your printMaybe, it is with printMaybe Nothing in your do block, since then it is unclear what the a type is.

You thus likely have something like:K

main = do
    putStrLn "====== Tests Start ======"
    putStrLn (printMaybe Nothing)

But it is unclear what the a of the type Maybe a is for Nothing. You can give a hint to the compiler with:

main = do
    putStrLn "====== Tests Start ======"
    putStrLn (printMaybe (Nothing :: Maybe Int))

How do I use putStrLn on Maybe value in Haskell Stack test?

1 Answers