Is there a way to script a ghci session?

Question

My goal is to pipe some steps for ghci to run from a bash script and then exit cleanly. The commentary online says to use runhaskell for this.

This is the command I'm trying to run:

ghci> import System.Random 

ghci> random (mkStdGen 100) :: (Int, StdGen) 

With expected result similar to:

(-3633736515773289454,693699796 2103410263)

When I drop this into a file randomtest.hs and execute it with runhaskell I get the following error:.

randomtest.hs:3:1: error:
    Invalid type signature: random (mkStdGen 100) :: ...
    Should be of form <variable> :: <type>

It seems I can't use the runhaskell method to blindly execute ghci inputs.

Now the way to work around this is to add extra commands to the file that is passed to runhaskell:

main = do print (random (mkStdGen 100) :: (Int, StdGen))

My goal is to automate the running of ghci work for a haskell course I'm using. I want to be able to run the ghci command from a bash script - in the format that ghci expects, and have it cleanly exit from ghci (or whatever runs it).

My question is: Is there a way to script a ghci session?

Answer 1

You'll want to use expect for that, it allows you to interactively control a REPL with simple commands. This script does what you want:

#!/usr/bin/env expect

log_user 0
spawn ghci
log_user 1

expect ".*> "
send ":set prompt \"ghci> \"\n"

expect "ghci> "
send "import System.Random\n"
expect "ghci> "
send "random (mkStdGen 100) :: (Int, StdGen)\n"

interact

Running this gives you the following:

$ ./ghci-interactive
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Prelude> :set prompt "ghci> "
ghci> import System.Random
ghci> random (mkStdGen 100) :: (Int, StdGen)
(-3633736515773289454,693699796 2103410263)
ghci> 

Note: You might need to adjust this a bit to be resistant to users setting the prompt in ~/.ghci.