This is GHCi plus extra goodies. It is a direct codebase copy from the GHC codebase of GHCi with modifications to be buildable as a Hackage package.

The features additional to normal GHCi function in the latest GHC (7.8.3) release are:

• The :set +c command: collect information about modules after they've been loaded, and remember it between loads (including failed ones). I recommend adding this line to your ~/.ghci file.

• The :type-at command (requires +c): show the type at the given position in the module. Example:

  *X> :type-at X.hs 6 6 6 7 f
  Int -> Int

  This can be useful to get the type of a pattern variable or an arbitrary selected expression.

  The parameters are:

    <filename> <line> <col> <end-line> <end-col> <text>
  

  text is used for when the span is out of date, i.e. the file changed and the code has moved, text can be used to confirm the contents of the span matches, and to fallback to a general :t-like lookup.

• The :loc-at command (requires +c): get the location of the thing at the given position in the module. Example:

  *X> :loc-at X.hs 6 14 6 16 mu
  X.hs:(8,7)-(8,9)

  This is useful for goto-definition features of editors and IDEs.

  Parameters are the same as for :type-at.

• The :all-types command (requires +c): list all types in the project: expressions, bindings top-level and local. Sort of like :browse on steroids.

  > :all-types
  ghc/GhciTypes.hs:(38,13)-(38,24): Maybe Id
  ghc/GhciTypes.hs:(45,10)-(45,29): Outputable SpanInfo
  ghc/GhciTypes.hs:(45,10)-(45,29): (Rational -> SpanInfo -> SDoc) -> Outputable SpanInfo

Columns start at 1 instead of zero, because this is how GHC now outputs its column numbers.

From source:

$ git clone [email protected]:chrisdone/ghci-ng.git
$ cabal install ghci-ng/

This version is not available on Hackage yet.

(Hackage flat out rejects the -dynamic flag which is required for GHCi to work properly.)

It should say GHCi-ng on the welcome message:

$ ghci-ng
GHCi-ng, version 7.8.2 [NG/0.0]: http://www.haskell.org/ghc/  :? for help

You're now using the next generation GHCi!

Run cabal repl in the following way:

$ cabal repl --with-ghc=ghci-ng

Using with haskell-mode is very easy, it's a drop-in replacement for the normal use of GHCi.

If you use the cabal-repl process type, use:

(setq haskell-process-args-cabal-repl
      '("--ghc-option=-ferror-spans" "--with-ghc=ghci-ng"))

Plain GHCi process type:

(setq haskell-process-path-ghci "ghci-ng")

Commands that use GHCi-ng:

• M-x haskell-mode-show-type-at
• M-x haskell-mode-goto-loc

Recommended bindings:

(define-key interactive-haskell-mode-map (kbd "M-.") 'haskell-mode-goto-loc)
(define-key interactive-haskell-mode-map (kbd "C-c C-t") 'haskell-mode-show-type-at)

Make sure to run :set +c in GHCi to enable collection of info, then load some modules and the above commands will work.

Patches welcome.

Patches welcome.

This project should ideally support the current major GHC version and the past major GHC version:

• GHC 7.8