This looks like a bug.

-hidir documentation says

> Redirects all generated interface files into ⟨dir⟩, instead of the default.
> Please also note that when doing incremental compilation (by ghc --make or ghc -c), this directory is where GHC looks into to find interface files.

And documentation for -i doesn't mention looking for interface files.

So by doing

% ghc -dynamic-too -c lib/Lib.hs -odir odir -hidir hidir_Lib
% ghc -dynamic-too -c Main.hs -odir odir -hidir hidir_Main -ihidir_Lib
1
% ghc odir/Lib.o odir/Main.o -o Demo                                 
% ./Demo                                                             
True

everything compiles, TH is run, and demo works. The result files are

% find .
.
./Demo
./Main.hs
./hidir_Main
./hidir_Main/Main.dyn_hi
./hidir_Main/Main.hi
./odir
./odir/Main.dyn_o
./odir/Main.o
./odir/Lib.dyn_o
./odir/Lib.o
./hidir_Lib
./hidir_Lib/Lib.dyn_hi
./hidir_Lib/Lib.hi
./lib
./lib/Lib.hs

---

The confusing error is caused by Lib module in libiserv:
https://hackage.haskell.org/package/libiserv
which GHC picks!

If we rename your Lib module to Library, the error is way better: Could not find module ‘Library’

I also tried using same -hidir when compiling both modules, then GHC still cannot find the Library interface, even the documentation says it should.

Please open a GHC issue at https://gitlab.haskell.org/ghc/ghc/-/issues

- Oleg

Dear devs,

I'm confused about the meaning of -hidir and -i. Here's my experiment with both ghc-9.2.0 and ghc-8.10.4.

> $ find

> ./Main.hs
> ./lib/Lib.hs

>

> $ ghc -dynamic-too -c lib/Lib.hs -odir odir -hidir hidir_Lib

>

> $ ghc -dynamic-too -c Main.hs -odir odir -ihidir_Lib -hidir hidir_Main

>

> Main.hs:3:1: error:
>     Bad interface file: hidir_Main/Lib.hi
>         hidir_Main/Lib.hi: openBinaryFile: does not exist (No such file or directory)
>   |
> 3 | import Lib
>   | ^^^^^^^^^^

If I only use -hidir, it still fails with another error

> $ ghc -dynamic-too -c Main.hs -odir odir -hidir hidir_Lib

>
> Main.hs:7:29: error: Variable not in scope: f
>   |
> 7 | main = print $(runIO (print f) >> [| True |])
>   |                             ^

If I use both -i and -hidir pointing to the same folder, then it works!

> $ ghc -dynamic-too -c Main.hs -odir odir -ihidir_Lib -hidir hidir_Lib

Now, is this behavior a bug or a feature? And if it is a bug, what is the expected behavior?

Programs copied below.

Thank you!

Facundo

> $ cat lib/Lib.hs
> module Lib where
>
> f :: Int
> f = 1

>

> $ cat Main.hs
> {-# LANGUAGE TemplateHaskell #-}
>
> import Lib
> import Language.Haskell.TH
>
> main :: IO ()
> main = print $(runIO (print f) >> [| True |])

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