Stack Build Cannot Find Local Library - haskell

Question

I successfully replicated this example using GHC by itself.
https://wiki.haskell.org/Calling_Haskell_from_C
The end goal is to write 99% of my program in Haskell, and then call it from an event loop written in C:
#include <HsFFI.h>
#ifdef __GLASGOW_HASKELL__
#include "../.stack-work/dist/x86_64-linux/Cabal-2.4.0.1/build/Lib_stub.h"
extern void __stginit_Lib(void);
#endif
#include <stdio.h>
#include <time.h>
extern void hs_add_root (void (*init_root)(void));
int main(int argc, char *argv[])
{
int i;
hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
hs_add_root(__stginit_Lib);
#endif
for (int m = 0; m < 10; ++m) {
i = fibonacci_hs(42);
printf("Fibonacci: %d\n", i);
}
hs_exit();
return 0;
}
The motivation to run an event loop in C is that, from what I've read, forcing evaluation X times/second is difficult or impossible in Haskell.
Here is package.yaml:
name: c-loop
version: 0.1.0.0
github: "githubuser/c-loop"
license: BSD3
author: "Author name here"
maintainer: "example#example.com"
copyright: "2019 Author name here"
extra-source-files:
- README.md
- ChangeLog.md
# Metadata used when publishing your package
# synopsis: Short description of your package
# category: Web
# To avoid duplicated efforts in documentation and dealing with the
# complications of embedding Haddock markup inside cabal files, it is
# common to point users to the README.md file.
description: Please see the README on GitHub at <https://github.com/githubuser/c-loop#readme>
dependencies:
- base >= 4.7 && < 5
library:
source-dirs: src
executables:
c-loop-exe:
main: Main.hs
source-dirs: app
ghc-options:
- -threaded
- -rtsopts
- -with-rtsopts=-N
- -fobject-code
# - -no-hs-main
- --make -no-hs-main -optc-O ./c/eventLoop.c Lib -o eventLoop
dependencies:
- c-loop
tests:
c-loop-test:
main: Spec.hs
source-dirs: test
ghc-options:
- -threaded
- -rtsopts
- -with-rtsopts=-N
dependencies:
- c-loop
When I run:
$ stack build
I get this:
<no location info>: error: module ‘Lib’ cannot be found locally
Does anyone know what is going on?

Answer 1

As an aside, your motivation seems misguided. I don't think there's any benefit to having a C event loop whose only purpose is to "force evaluation" on a regular schedule. You can do that in Haskell just fine.
What's going wrong in your example above is probably the Lib in the ghc-options. However, there are other Cabal fields you should be using instead which will make things work more smoothly.
Here's how to get your minimal example working with Stack. Create a fresh directory with the four files listed below and run stack build, then stack exec c-loop-exe.
A few points:
You might be able to do this with a package.yaml file, but you'll have to convert the Cabal syntax over.
You don't need all that __stginit and hs_add_root garbage anymore, unless you're using GHC < 7.2.
You don't need to hard-code the path for the stub, if you set up the Cabal file correctly (i.e., using c-sources).
The -opt-O2 flag is unnecessary. It's the Stack default.
The contents for the four files:
-- stack.yaml
resolver: lts-13.21
packages:
- .
-- c-loop.cabal
cabal-version: 1.12
name: c-loop
version: 0.1.0.0
build-type: Simple
executable c-loop-exe
main-is: src/Lib.hs
ghc-options: -no-hs-main
c-sources: c/eventLoop.c
build-depends:
base >=4.7 && <5
default-language: Haskell2010
-- c/eventLoop.c
#include <stdio.h>
#include <time.h>
#include "HsFFI.h"
#include "Lib_stub.h"
int main(int argc, char *argv[])
{
int i;
hs_init(&argc, &argv);
for (int m = 0; m < 10; ++m) {
i = fibonacci_hs(42);
printf("Fibonacci: %d\n", i);
}
hs_exit();
return 0;
}
-- src/Lib.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Lib where
import Foreign.C.Types
fibonacci :: Int -> Int
fibonacci n = fibs !! n
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
foreign export ccall fibonacci_hs :: CInt -> CInt