Discuss `Show` instance for `Annotation tag`

Clarification and motivation

There's an unwritten expectation that the output of show should be a string you can copy-paste back into the editor. That is what stock instances do, and what haskell devs usually expect to see:

Derived instances of Show have the following properties, which are compatible with derived instances of Read:

The result of show is a syntactically correct Haskell expression containing only constants (...)

The ability to be able to show a value and copy-paste it into the editor is extremely useful while debugging.

However, the Show instance for Annotation tag doesn't do this. It pretty-prints the annotation:

λ> [fieldAnnQ|a|]
%a

Furthermore, this can lead to confusing scenarios. For example, the Show instance for Notes t outputs a "syntactically correct Haskell expression" with pretty-printed (syntactically incorrect) annotations in the middle:

λ> NTPair @'T.TInt @'T.TInt noAnn [fieldAnnQ|a|] noAnn [varAnnQ|b|] noAnn starNotes starNotes
NTPair %a @b (NTInt) (NTInt)

Notice how NTInt's field and 3 of NTPair's fields are completely missing from the output.

For those reasons, I think we should just use a stock-derived instance for Annotation tag.

On the other hand, the stock derived instance will not show the phantom type tag, which can also be useful. So perhaps printing one of these would be a fair compromise:

  1. Print a type application: UnsafeAnnotation @tag "ann"
  2. Print a quasiquote: [fieldAnnQ|ann|]

They're both copy-pastable syntactically correct Haskell expressions.

Thoughts?

Also, slightly related topic: The output of pretty for Notes t instance is not very... pretty?

λ> pretty $ NTPair @'T.TInt @'T.TInt noAnn [fieldAnnQ|a|] noAnn [varAnnQ|b|] noAnn starNotes starNotes
NTPair %a @b (NTInt) (NTInt)

I suggest changing it to build = build . mkUType, which would print:

pair (int %a @b) int

Acceptance criteria

  • We've decided what to do about the Show instance for Annotation tag