Quick Haskell Trick

• May 4, 2020
• haskell  
• ghci  
• short-iteration  

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.

We can do better though, ghci lets you define your custom commands. Here’s the incantation:

ghci> :def g \_ -> return ":r\n:main"

Now you can just:

ghci> :g

and it will :reload and run :main for you.

It’s not smart enough to understand when :reload fails, but that’s OK because if :reload fails there’s no main and :main fails as well.