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: