I usually have 'Use Show instance' switched off (aka use 'built-in printer'), so I can see (at least in crude form) imported or 'system' datatypes that don't have show instances/also it's a bit more revealing if I get exceptions. I have a datatype:
data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) deriving (Eq, Show, Read)
For results of expressions that were `Node`s, the built-in printer just ignored the first field. (`show`/print of those values works fine.)
FoldableTree> myTree <> fdmap toUpper myTree Node 'c' Empty
FoldableTree> print $ myTree <> fdmap toUpper myTree Node (Node Empty 'A' (Node Empty 'C' (Leaf 'a'))) 'c' Empty
Most disconcerting until I realised what was going on. (Of course I don't expect the built-in printer to produce pretty output). AntC