[Haskell-cafe] Using ghc-mod with FFI

Albert Y. C. Lai trebla at vex.net
Mon Dec 1 23:54:33 UTC 2014

On 2014-11-28 05:11 PM, Dan Piponi wrote:
> In my code I have this line:
> foreign import ccall "set_num_states" setNumStates :: Int -> IO ()
> I can build my project using cabal and have an appropriate 
> "c-sources:" line.
> But when I use ghc-mod check (eg. from " ghc-mod-", and 
> launched from Syntastic) it complains:
> "ghc-mod:
> ByteCodeLink: can't find label
> During interactive linking, GHCi couldn't find the following symbol:
>   set_num_states"
> How should I be checking my code?

I don't know about ghc-mod. But I know ghci, and you may be able to lift 
ghci things to ghc-mod things.

"ghci M.hs" or "ghci" and then ":load M.hs" gives the same error. 
However, you're expected to:

(if your C file is s.c)

"ghci s.o M.hs" or "ghci M.hs s.o" or "ghci s.o" and then ":load M.hs"

that's how ghci knows where to find set_num_states.

See also GHC user's guide section 2.6.2.

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