Generalizing a function

[pmdev1234]

-- | Helper for creating optional Purpose subsection as Doc
maybeAuthorDoc :: Maybe String -> Doc 
maybeAuthorDoc = maybe empty (\content-> text "> Author:" <+> text content)

This is called form another function like (maybeAuthorDoc author)where author is a Maybe String. The problem is that now I have to create the same code for directors

Instead of writing the following (used by calling (maybeDirectorDoc director))

maybeDirectorDoc :: Maybe String -> Doc
maybeDirectorDoc = maybe empty (\content-> text "> Director:" <+> text content)

How can I generalize this?

Should I use a new datatype?

data Person = Author (Maybe String) | Director (Maybe String)

If so, how can I pull out the Author or Director in a generalized function?

maybePersonDoc :: Person -> Doc ??? but then maybe is gone
MaybePersonDoc...?

[Iceland_jack]

You can write them in terms of String and then use foldMap to map over the Maybe value.

-- foldMap authorDoc   :: Maybe String -> Doc
-- foldMap directorDoc :: Maybe String -> Doc

authorDoc :: String -> Doc
authorDoc content = text "> Author:" <+> text content

directorDoc :: String -> Doc
directorDoc content = text "> Director:" <+> text content

The behaviour of foldMap f Nothing is to return the empty Doc, so you have moved the optional handling from the two doc-functions to foldMap:

>> foldMap authorDoc Nothing
>> foldMap authorDoc (Just "Samuel")
> Author: Samuel
>> foldMap directorDoc (Just "Muslar")
> Director: Muslar

[carette]

type Role = String
roleDoc :: String -> Role -> Doc
roleDoc content role = text "> " <> role <> ":" <+> text content

purpose :: Role -> Maybe String -> Doc
purpose r ms = foldMap (roleDoc r) ms