Haskell GHCI, can't seem to enter interactive execution of Main module

Question

I need to debug some haskell code and figure out where an issue is occuring at runtime. When I run my program is says this

main: Prelude.(!!): index too large

which leads me to believe that there is a place I'm sending a list to a function that I really shouldn't be, and I don't know where. This is a 600+ line semantic analysis program. Trouble is, I can't for the life of me figure out where this !! error is. The debugging documentation is really confusing, and I can't get GHCI to cooperate so I can maybe figure this out. Whenever I run ghci main.hs, this happens:

GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Ok, modules loaded: Main.
Prelude Main>

I cannot seem to get it to turn to

*Main>

so that I can actually set breakpoints and all that fun stuff. How do I get this to work?

Answer 1

The former line means you are loading the file in compiled mode. You can fix this by making sure that there is no compiled version of the file around; or you can tell ghci to use interpreted mode by prepending a * when loading the file:

Prelude Main> :l *Main
*Main>

(Or run ghci '*Main.hs' -- adapted to your shell's escaping needs as appropriate.)