Currently, :set -ddump-json in GHCi causes all errors to be gathered to be written out on exit. ghci -ddump-json is even worse, only resulting in a [] being output on exit.

However, I believe this completely defeats the purpose of this flag in GHCi. I expect ghci -ddump-json to work like this: for each evaluation typed at the GHCi prompt, one JSON array is output for all messages this evaluation caused.

This proposed behavior is consistent with other flags like -ddump-simpl, and will allow tooling to interact with GHCi nicely.

Reporting this as a bug because this looks half-supported rather than unsupported, and also because of the different behaviors of ghci -ddump-json and :set -ddump-json. Sorry if it was not warranted.

Steps to reproduce

cmd> is the shell prompt.

cmd> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.2.1

cmd> type tmp.hs
main = print 2 + 2

cmd> ghci -ddump-json tmp.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( tmp.hs, interpreted )

tmp.hs:1:8: error:
    ? No instance for (Num (IO ())) arising from a use of ‘+’
    ? In the expression: print 2 + 2
      In an equation for ‘main’: main = print 2 + 2
  |
1 | main = print 2 + 2
  |        ^^^^^^^^^^^
Failed, 0 modules loaded.
Prelude> :q
Leaving GHCi.
[]

cmd> ghci
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Prelude> :set -ddump-json
Prelude> :l tmp.hs
[1 of 1] Compiling Main             ( tmp.hs, interpreted )

tmp.hs:1:8: error:
    ? No instance for (Num (IO ())) arising from a use of ‘+’
    ? In the expression: print 2 + 2
      In an equation for ‘main’: main = print 2 + 2
  |
1 | main = print 2 + 2
  |        ^^^^^^^^^^^
Failed, 0 modules loaded.
Prelude> :q
Leaving GHCi.
[
 {"span": {"file": "tmp.hs","startLine": 1,"startCol": 8,"endLine": 1,"endCol": 19},"doc": "\u2022 No instance for (Num (IO ())) arising from a use of \u2018+\u2019\n\u2022 In the expression: print 2 + 2\n  In an equation for \u2018main\u2019: main = print 2 + 2","severity": "SevError","reason": null},

 {"span": null,"doc": "[1 of 1] Compiling Main             ( tmp.hs, interpreted )","severity": "SevOutput","reason": null}]