The Haskell REPL (Read Print Eval Loop) - GHC interpreter, ghci invoked with:

$ ghci

or:

$ stack repl

or:

$ cabal new-repl 

or:

$ cabal repl

is an environment enabling fast iteration.

You can test a particular function and that’s cool and all, but you can also run an entire program in the interpreter. You do that with:

ghci> :main

or

ghci> main

The flow is as follows:

1. You make some changes in your editor

• You :r or :reload
• You make some more changes to make it compile
• You :r again
• You run :main to play around with your program
• You Ctrl-C to kill the program
• Go back to step one

What’s a little inefficient is that to run your new version of a program you have to issue two commands: :r and :main.