Debugging with GHCi.

[Lemmih]

Greetings,

A few days ago I wondered whether it would be possible to call GHCi
from interpreted byte-code. It turned out that it was, and it was even
fairly easy. Here's a preliminary result:

Test.lhs:
> import GHC.Base
> run :: Int -> ()
> run i
>     = let b = False
>           c x = x + a + i
>       in breakPoint ()
>       where d = "str"
>             a = 10
> runIO :: IO String
> runIO = do putStr "Enter line: "
>            line1 <- getLine
>            breakPoint $ do
>            putStr "Enter another line: "
>            line2 <- getLine
>            return (unwords [line1,line2])


Output from a GHCi session:

ghc/compiler/stage2/ghc-inplace --interactive Test.lhs -v0
*Main> run 100
Local bindings in scope:
  a :: Int, c :: Int -> Int, b :: Bool, i :: Int, d :: [Char]
Test.lhs:6> (a, b, c 10, d)
(10,False,120,"str")
Test.lhs:6> :q
Returning to normal execution...
()
*Main> runIO
Enter line: Hello
Local bindings in scope:
  line1 :: String
Test.lhs:12> map Char.toUpper line1
"HELLO"
Test.lhs:12> :q
Returning to normal execution...
Enter another line: World
"Hello World"
*Main> :q

Note that the problems with the representation of variables and their
laziness has been delegated to the user. Has this approach been tried
before?

Simon PJ: You mentioned something about some ideas for building a
debugger into GHCi in your call for interns two months ago. Care to
elaborate on those or can I perhaps reach them online?

--
Friendly,
  Lemmih