How to abort execution in GHCI?

Question

When I launch

ghci> last [0..]

I can interrupt it with Ctrl+C.

However

ghci> last (repeat 0)

cannot be aborted with Ctrl+C. GHCI silently ignores the keystrokes.

How to abort this command in GHCI? Is it a bug?

Answer 1

(Caveat lector: I use Linux, and run zsh on urxvt or gnome-terminal. If you use a different operating system, terminal, or shell, it's possible this will work differently for you.)

The way I usually handle this is to hit Ctrl+Z (which puts it in the background, pausing execution entirely as a side-effect) then kill the job. Usually this is kill %1, though you can run jobs to double-check.

You can also start a new terminal and do something like killall -9 ghci, but this has a much higher resource cost: you are spawning a few new processes, opening X connections, doing whatever it is your terminal does when it initializes itself, doing whatever it is your shell does when it initializes itself, etc. If you're in the situation I often find myself in -- ghci is swapping like crazy -- that just gives ghci more time to screw things up.

If you can predict this problem, and are compiling, you can use -fno-omit-yields to ask GHC to insert Ctrl+C checks even inside tight, non-allocating loops.

Answer 2

You can quit GHCI using :quit command

Prelude> :quit
Leaving GHCi.

Or by pressing Control+D that sends EOF signal.

If GHCI is busy you have no choice other than killing the process manually.

Answer 3

Ctrl+Z seems to leave the shell, but it doesn't entirely close the process.

For Example

$ ghci
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> 
[4]+  Stopped                 ghci
$ ps
  PID TTY          TIME CMD
 3160 pts/1    00:00:00 bash
 3554 pts/1    00:00:21 emacs
 5602 pts/1    00:00:00 ghc
 5693 pts/1    00:00:00 ps

However if you do Ctrl+D

$ ghci
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> 
Leaving GHCi.
$ ps
  PID TTY          TIME CMD
 3160 pts/1    00:00:00 bash
 3554 pts/1    00:00:21 emacs
 5870 pts/1    00:00:00 ps

So the proper way to close haskell-shell is to hit Ctrl+D.

Note: Tested on Linux (Ubuntu 16.04 LTS)