This is a linking issue. It seems GHC 7 automatically feeds the linker SimpleJSON.o so when you explicitly provide it too then you get those conflicts. All you need to do is call:

ghc -o simple Main.hs

Unless you're using GHC 6, then the original command is correct:

ghc -o simple Main.hs SimpleJSON.o

Or even better, use the --make flag as that works with either version:

ghc --make -o simple Main.hs

Cheers, Thomas On Wed, Aug 17, 2011 at 10:37 AM, Paul Reiners wrote:

I'm trying to do the following from Chapter 5 of "Real World Haskell":

Our choice of naming for the source file and function is deliberate. To create an executable, ghc expects a module named Main that contains a function named main. The main function is the one that will be called when we run the program once we've built it. 6 comments

ghc -o simple Main.hs SimpleJSON.o

---from http://book.realworldhaskell.org/read/writing-a-library-working-with-json-da...

When I do that, I get this error:

C:\ch05>ghc -o simple Main.hs SimpleJSON.o [2 of 2] Compiling Main             ( Main.hs, Main.o ) Linking simple.exe ... SimpleJSON.o:fake:(.data+0x0): multiple definition of `SimpleJSON_getArray_closure' .\SimpleJSON.o:fake:(.data+0x0): first defined here SimpleJSON.o:fake:(.text+0x54): multiple definition of `SimpleJSON_getArray_info' .\SimpleJSON.o:fake:(.text+0x54): first defined here SimpleJSON.o:fake:(.data+0x4): multiple definition of `SimpleJSON_getObject_closure' .\SimpleJSON.o:fake:(.data+0x4): first defined here

What's going wrong here?

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