Pprintast omits parens for List ((String "A")::[])

[vicuna]

Original bug ID: 6546
Reporter: @whitequark
Assigned to: @gasche
Status: closed (set by @xavierleroy on 2016-12-07T10:34:48Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.0+beta1 / +rc1
Target version: 4.02.2+dev / +rc1
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @gasche @hcarty

Bug description

It is printed back as

`List (`String "A")::[]

[vicuna]

Comment author: @gasche

I cannot reproduce this using the -dsource flag (which, if I understand correctly, is what uses Pprintast).

My test file test2.ml is the following:
let f x = List ((String "A"):: [])
let f x = List ((String "A")::x)

On my machine, "ocamlc -dsource test2.ml" under both 4.01.0 and the current 4.02 branch prints the following, which seems correct:

let f x = List [String "A"]
let f x = List ((String "A") :: x)

[vicuna]

Comment author: @whitequark

Oh, it seems this only happens in patterns. E.g. try:

match (assert false) with `List ((`String "tea")::x::[]) -> ()

[vicuna]

Comment author: @gasche

Thanks, fixed in 4.02.

The pprintast code is doing the non-trivial job of guessing parenthesis and syntactic resugaring, and not really pleasant to work with. The current codebase does not handle resugaring "::" in the same way in expression and patterns, and one may wonder why, but "if it ain't broke, don't fix it" -- we have better fish to fry.