How to exploit ./hadrian/ghci to find errors quickly?

[Tom Ellis]

On Fri, Jan 28, 2022 at 11:52:00AM -0500, Norman Ramsey wrote:
>  > My recommendation: ./hadrian/ghci. 
> 
> I'm about to change a type definition.  This change may wreak havoc in
> many parts of GHC, and this is exactly what I want: I'm looking for
> type-error messages that will tell me what code I need to fix.
> I do this work in emacs using `M-x compile` and `next-error`.  The key
> property is that `next-error` requires just a couple of keystrokes
> (C-x `) and it makes Emacs take me to the relevant source-code
> location quickly.
> 
> But telling `M-x compile` to run `./hadrian/build` is quite slow.  Is
> there a way to leverage `./hadrian/ghci` for this workflow?

Are you familiar with dante-mode for Emacs?  It relies on ghci behind
the scenes to point out type errors in Emacs Haskell buffers.  You
might find it convenient even if only for this single-purpose use
case.  I wrote up how I use dante:

http://h2.jaguarpaw.co.uk/posts/how-i-use-dante/

You will have to `M-x customize-group dante` to set the GHCi path to
`.../hadrian/ghci`.

Tom