How to stop Maybe from being passed into another function in Haskell

Question

I have already looked at this answer, but it is unclear to me how to apply it in my situation.

I have the following function:

designC :: DesignC -> Maybe CDef -> String
designC FirstString _ = "first_string"
designC SecondString _ = "second_string"
designC UnknownString c = getName c

The getName function takes a CDef as it's argument. However the above code will not work and has the following error:

No instance for (getIdea (Maybe CDef))     
arising from a use of ‘getName’
In the expression: getName c

I tried:

designC UnknownString c = getName (Just c)

but the error shows that this made things worse

No instance for (getIdea (Maybe (Maybe CDef)))

How can I pass just the CDef to getName?

What do you expect `designC` to do if the `Maybe CDef` is `Nothing`? Should it throw an error, propagate the `Nothing` along or return a default value? — Vikstapolis, Feb 13 '22 at 06:02
Are we supposed to know what `getName` and `getIdea` [sic] are? Anyway, I don't. Show us their definitions, please. — Daniel Wagner, Feb 13 '22 at 06:22
What value exactly do you want `designC UnknownString Nothing` to return? — Joseph Sible-Reinstate Monica, Feb 13 '22 at 08:17

score 2 · Accepted Answer · answered Feb 13 '22 at 07:54

You have to handle a situation when your Maybe is Nothing.

Either provide some default value

defaultValue = ""

designC :: DesignC -> Maybe CDef -> String
designC FirstString _ = "first_string"
designC SecondString _ = "second_string"
designC UnknownString (Just c) = getName c
designC UnknownString Nothing = defaultValue

-- or shorter
designC UnknownString c = maybe defaultValue getName c

Or raise an exception

data MyException = VeryNastyError

designC :: MonadError MyException m => DesignC -> Maybe CDef -> m String
designC FirstString _ = pure "first_string"
designC SecondString _ = pure "second_string"
designC UnknownString (Just c) = pure $ getName c
designC UnknownString Nothing = throwError VeryNastyError

How to stop Maybe from being passed into another function in Haskell

1 Answers1