How to export a (stack) GHCi session

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Where does stack ghci store its history? Per this question it ought to be in ~/.ghc/ghci_history but that does not seem to be the target for all GHCi sessions invoked by stack within a project. (There surely must be concurrency issues if all sessions were writing to that file?) The user guide mentions the option -flocal-ghci-history but that is not invoked via stack by default.

The use case is: You have a project and tinker with some modules interactively. After a while you realize that you have done something useful in the session and would like to save the commands somewhere.

Obvious work-arounds are:

  1. Alias stack ghci to stack ghci --ghci-options -flocal-ghci-history and inspect the .ghci_history file if needed
  2. Start with an empty .hs file of your choice and keep re-compiling it (in the IDE of your choice)

But GHCi does always keep a (Haskeline?) history, so how do I get my hands on it from within a vanilla GHCi session?

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For GHCi itself, there is this:

For Stack, it just Rio.Process.exec’s the specified GHC executable with --interactive and other GHC flags/options. That is, I think Stack is just relying on that executable’s handling of concurrency issues (if any).

From experimentation:

  • GHCi reads from that history file when it starts; and
  • writes to the file with additions when it finishes; and
  • over-writes any file that has been written to by other GHCi which started and finished in the interim.
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So that was my misconception: There is no trace of a still runnnig session’s history on disk. Unfortunately the history seems to be limited to 100 lines, whence one should not rely on the history alone.

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The command line history length is, I think, configurable:

There is an open Haskeline issue, from 2014, for capturing history on a line-by-line basis:

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Thanks, the linked issue summarizes the difficulties pretty well, that need to be overcome to persist the history on-the-fly. Thus it seems my work-around #2 is the safe way to go: Do not experiment in an interactive session when there is a chance you’ll need to persist the commands.

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