Haskell Language Using GHCi Breakpoints with GHCi
Example
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>
