Question

Suppose we have the following:

data Foo x
from_list :: [x] -> Foo x
to_list :: Foo x -> [x]

Suppose I want to declare instance (Show x) => Show (Foo x) such that showing a value produces the appropriate call to from_list. How exactly do I do that? In particular, how do I implement showsPrec so that the trixy fiddly precedence rules are satisfied? (That is, put the expression in brackets if and only if it’s necessary.)

Answer 1

What you want here is basically the same as what Data.Set does, so we can just modify that a little:

instance Show x => Show (Foo x) where
  showsPrec p xs = showParen (p > 10) $
    showString "from_list " . shows (to_list xs)

In other words, we use the Show instance for lists, prepend "from_list " to that, and use showParen to add brackets around it if the surrounding context has higher precedence than function application (which has precedence 10).

> show (from_list [1, 2, 3])
"from_list [1,2,3]"
> show (Just $ from_list [1, 2, 3])
"Just (from_list [1,2,3])"