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Write a C function and Call the C function from Haskell

1. Write your C function in AronCLibFFI.c

  
  void print_ascii(){
	  for(int i = 0; i < 127; i++){
		printf("%d 0x%x %c\n", i, i, i);
	  }
  }

2. Add AronCLibFFI.c full path in your cabal file: haskellffi.cabal

  
  executable haskellffi 
  hs-source-dirs:      src
  main-is:             Main.hs
  default-language:    Haskell2010
                                                                  + ---> contains print_ascii()
                                                                  ↓
  c-sources:           /Users/aaa/myfile/bitbucket/haskellffi/AronCLibFFI.c
                                                                  ↑
                                                                  + ---> Main.c can find your C function
  build-depends:       base >= 4.7 && < 5,

3. In your Main.hs, write haskell function to match the C function in AronCLibFFI.c

  
  foreign import ccall "print_ascii" 
	  c_print_ascii :: IO() 

  f_print_ascii::IO()
  f_print_ascii = c_print_ascii

4. Call your Haskell function in your Main.hs

  
  Main = f_print_ascii

5.How to call your FFI function inside GHCi,You need to load the C Object file (AronCLibFFI.o) when you start GHCi


  stack ghci --ghci-options="/Users/aaa/myfile/bitbucket/haskellffi/AronCLibFFI.o"

How Haskell call C function

-- Here is simple code for FFI
--
import Foreign
import Foreign.C.Types

foreign import ccall unsafe "math.h sin"
     c_sin :: CDouble -> CDouble

main = do 
        let c = c_sin 3.1415 
        print c

C String to Haskell String, Haskell String to C String

  peekCString::CString -> IO String
  newCString::String -> IO CString

Conversion between Haskell Type and C Type