Using GHCi

Remarks#

GHCI is the interactive REPL that comes bundled with GHC.

Starting GHCi

Type ghci at a shell prompt to start GHCI.

$ ghci
GHCi, version 8.0.1: https://www.haskell.org/ghc/  :? for help
Prelude> 

Changing the GHCi default prompt

By default, GHCI’s prompt shows all the modules you have loaded into your interactive session. If you have many modules loaded this can get long:

Prelude Data.List Control.Monad> -- etc

The :set prompt command changes the prompt for this interactive session.

Prelude Data.List Control.Monad> :set prompt "foo> "
foo> 

To change the prompt permanently, add :set prompt "foo> " to the GHCi config file.

The GHCi configuration file

GHCi uses a configuration file in ~/.ghci. A configuration file consists of a sequence of commands which GHCi will execute on startup.

$ echo ":set prompt \"foo> \"" > ~/.ghci
$ ghci
GHCi, version 8.0.1: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from ~/.ghci
foo> 

Loading a file

The :l or :load command type-checks and loads a file.

$ echo "f = putStrLn \"example\"" > example.hs
$ ghci
GHCi, version 8.0.1: https://www.haskell.org/ghc/  :? for help
ghci> :l example.hs
[1 of 1] Compiling Main               ( example.hs, interpreted )
Ok, modules loaded: Main.
ghci> f
example

Quitting GHCi

You can quit GHCi simply with :q or :quit

ghci> :q
Leaving GHCi.

ghci> :quit
Leaving GHCi.

Alternatively, the shortcut CTRL+D (Cmd+D for OSX) has the same effect as :q.

Reloading a already loaded file

If you have loaded a file into GHCi (e.g. using :l filename.hs) and you have changed the file in an editor outside of GHCi you must reload the file with :r or :reload in order to make use of the changes, hence you don’t need to type again the filename.

ghci> :r
OK, modules loaded: Main.

ghci> :reload
OK, modules loaded: Main.

Breakpoints with GHCi

Multi-line statements

The :{ instruction begins multi-line mode and :} ends it. In multi-line mode GHCi will interpret newlines as semicolons, not as the end of an instruction.

ghci> :{
ghci| myFoldr f z [] = z
ghci| myFoldr f z (y:ys) = f y (myFoldr f z ys)
ghci| :}
ghci> :t myFoldr
myFoldr :: (a -> b -> b) -> b -> [a] -> b