Using GHCi

Breakpoints with GHCi

GHCi supports imperative-style breakpoints out of the box with interpreted code (code that’s been :loaded).

With the following program:

-- mySum.hs
doSum n = do
  putStrLn ("Counting to " ++ (show n))
  let v = sum [1..n]
  putStrLn ("sum to " ++ (show n) ++ " = " ++ (show v))

loaded into GHCi:

Prelude> :load mySum.hs
[1 of 1] Compiling Main             ( mySum.hs, interpreted )
Ok, modules loaded: Main.
*Main>

We can now set breakpoints using line numbers:

*Main> :break 2
Breakpoint 0 activated at mySum.hs:2:3-39

and GHCi will stop at the relevant line when we run the function:

*Main> doSum 12
Stopped at mySum.hs:2:3-39
_result :: IO () = _
n :: Integer = 12
[mySum.hs:2:3-39] *Main>

It might be confusing where we are in the program, so we can use :list to clarify:

[mySum.hs:2:3-39] *Main> :list
1  doSum n = do
2    putStrLn ("Counting to " ++ (show n))   -- GHCi will emphasise this line, as that's where we've stopped
3    let v = sum [1..n]

We can print variables, and continue execution too:

[mySum.hs:2:3-39] *Main> n
12
:continue
Counting to 12
sum to 12 = 78
*Main>

Starting GHCi

Type ghci at a shell prompt to start GHCI.

$ ghci
GHCi, version 8.0.1: http://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: http://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: http://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.

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

Remarks

GHCI is the interactive REPL that comes bundled with GHC.