Add custom type errors in some places

[chshersh]

This is a meta-issue to collect all places where custom type errors can be introduced to increase the quality of error messages and make relude more beginner-friendly

• Foldable1 for ordinary list
• ToString/ToText/ToLText/ConvertUtf8 for text data types: suggest proper function.
• head/tail/init/last
• lines/unlines/words/unwords
• error

Any other ideas? 🙂

Also, open question: is it worth to add type-errors-pretty to dependencies to write these errors?

[chshersh]

Looks like it's possible to have better custom type errors for functions like head, init, tail and last. Using the following code:

type IsNonEmpty (f :: Type -> Type) =
    (f ~ NonEmpty, CheckNonEmpty f)

type family CheckNonEmpty (f :: Type -> Type) :: Constraint where
    CheckNonEmpty NonEmpty = ()
    CheckNonEmpty [] = TypeError ('Text "Should be NonEmpty, not ordinary list")
    CheckNonEmpty _ = TypeError ('Text "Should be NonEmpty list")

head :: IsNonEmpty f => f a -> a
head = NE.head

I didn't expect this work. But looks like it works. We only need to write better error messages)