[Template-haskell] Calling foreign functions in a splice doesn't work

[Simon Peyton-Jones]

OK, I reckon I've fixed this.  You can now specify any extra C libraries =
on the command line (c_double.o), and they'll get linked if any TH runs =
happen.

Simon

| -----Original Message-----
| From: template-haskell-admin@haskell.org =
[mailto:template-haskell-admin@haskell.org] On Behalf Of
| Andr=E9 Pang
| Sent: 12 May 2003 16:14
| To: template-haskell@haskell.org
| Cc: chak@cse.unsw.edu.au
| Subject: [Template-haskell] Calling foreign functions in a splice =
doesn't work
|=20
| Hi all,
|=20
| I've been trying to call a foreign function (i.e. via the FFI) inside =
a
| splice, which doesn't seem to work.
|=20
| As an example, I'll attach a small program which does work, and then
| modify it so it doesn't work.  So, here's a program which does work:
|=20
| Splicer.hs:
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| > module Main where
| >
| > import Templates
| >
| > main :: IO ()
| > main =3D do
| >    putStrLn (show ($(doubleWrapper) 5))
|=20
| Templates.hs:
|=20
| > module Templates where
| >
| > import Language.Haskell.THSyntax
| >
| > doubleWrapper :: Expr
| > doubleWrapper =3D [| \x -> myDouble x |]
| >
| > myDouble :: Int -> Int
| > myDouble x =3D x * 2
|=20
| Pretty simple: the program splices in an Expr named doubleWrapper;
| doubleWrapper in turn calls myDouble, which is a simple function of
| type Int -> Int.  Execution:
|=20
| > 20:10 ~/th/ffi-bug % ghc -fglasgow-exts --make Splicer.hs
| > ...
| > 20:10 ~/th/ffi-bug % ./a.out
| > 10
|=20
| So that works.
|=20
| Here's a version of the program which modifies the 'myDouble' function
| so that it's now a foreign C function rather than a Haskell function.
| (Note that the main function and doubleWrapper functions are exactly
| the same):
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| SplicerFFI.hs:
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| > module Main where
| >
| > import TemplatesFFI
| >
| > main :: IO ()
| > main =3D do
| >    putStrLn (show ($(doubleWrapper) 5))
|=20
| TemplatesFFI.hs:
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| > module TemplatesFFI where
| >
| > import Language.Haskell.THSyntax
| >
| > doubleWrapper :: Expr
| > doubleWrapper =3D [| \x -> myDouble x |]
| >
| > foreign import ccall safe "c_double.h myDouble"
| >   myDouble :: Int -> Int
|=20
| c_double.h:
|=20
| > int myDouble (int x);
|=20
| c_double.c:
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| > int myDouble (int x)
| > {
| >    return x * 2;
| > }
|=20
| Compiling the FFI version of this small program doesn't work:
|=20
| > 20:20 ~/th/ffi-bug % gcc -c c_double.c
| > 20:20 ~/th/ffi-bug % ghc -fglasgow-exts --make c_double.o
| > SplicerFFI.hs c_double.o
| > Chasing modules from: SplicerFFI.hs
| > Compiling TemplatesFFI     ( TemplatesFFI.hs, ./TemplatesFFI.o )
| > Compiling Main             ( SplicerFFI.hs, ./SplicerFFI.o )
| > Loading package base ... linking ... done.
| > Loading package haskell98 ... linking ... done.
| > Loading package haskell-src ... linking ... done.
| >
| > TemplatesFFI.o: unknown symbol `_myDouble'
| >
| > SplicerFFI.hs:7:
| >     Exception when trying to run compile-time code:
| >         Code: doubleWrapper
| >         Exn:
| >     In the first argument of `show', namely
| >         `($[splice]doubleWrapper 5)'
| >     In the first argument of `putStrLn', namely
| >         `(show ($[splice]doubleWrapper 5))'
| >     In the result of a 'do' expression:
| >         putStrLn (show ($[splice]doubleWrapper 5))
|=20
| I suspect the compilation isn't working because the 'myDouble' symbol
| in c_double.o isn't being loaded _at splice time_ into the list of
| symbols that GHC knows about -- which is also why the exception =
occurs.
|   I've tried all sorts of kludges to convince GHC to load the symbol
| from the foreign object file, like doing "ar rcs libcdouble.a
| c_double.o" and then putting -L. -lcdouble on the commandline, passing
| additional linker/compiler flags with -optl and -optc, etc, and none =
of
| them have worked so far.
|=20
| This was tested with GHC on Mac OS X built from CVS 9 March 2003; my
| apologies for a fairly out-of-date GHC build, but I'm having trouble
| building the recent GHCs on Mac OS X.  Can somebody verify that this
| also occurs on Linux with a recent build of GHC, and hopefully fix it
| or give me a clue on where to start looking for the bug?
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| Thanks very much!
|=20
|=20
| --
| % Andre Pang : just.your.average.bounty.hunter
|=20
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