Fails with GHC-7.8.4 and installed transformers

[phadej]

% cabal new-build -w ghc-7.8.4 --constraint='transformers installed'
Build profile: -w ghc-7.8.4 -O1
In order, the following will be built (use -v for more details):
 - free-5.0.2 (lib) (first run)
Preprocessing library for free-5.0.2..
Building library for free-5.0.2..

In file included from src/Control/Applicative/Free.hs:9:0: 

include/free-common.h:15:0:
     error: missing binary operator before token "("
     #if MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)

[RyanGlScott]

See my comments here as to why this is happening.

[phadej]

@RyanGlScott I don't see how that is related. Note that in recent #172 transformers-compat dependency is removed completely.

[RyanGlScott]

Let's take a closer look at the CPP:

#ifndef MIN_VERSION_base
#define MIN_VERSION_base(x,y,z) 1
#endif

#ifndef MIN_VERSION_mtl
#define MIN_VERSION_mtl(x,y,z) 1
#endif

#if MIN_VERSION_base(4,9,0)
...
#else
#if MIN_VERSION_transformers(0,5,0)
...
#else
#if MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)
...
#endif
#endif
#endif

GHC 7.8.4 comes bundled with base-4.7, so that first conditional (#if MIN_VERSION_base(4,9,0)) will fail. Then, it'll try the next conditional (#if MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)). But as you've noted, free no longer depends on transformers-compat, so this will fail to preprocess, as MIN_VERSION_transformers_compat is no longer defined.

In that case, I have to wonder if this CPP needs to be updated to reflect transformers-compat's removal?

[phadej]

diff --git a/include/free-common.h b/include/free-common.h
index 220394d..fdaa8bf 100644
--- a/include/free-common.h
+++ b/include/free-common.h
@@ -12,8 +12,5 @@
 #if MIN_VERSION_transformers(0,5,0)
 #define LIFTED_FUNCTOR_CLASSES 1
 #else
-#if MIN_VERSION_transformers_compat(0,5,0) && !MIN_VERSION_transformers(0,4,0)
-#define LIFTED_FUNCTOR_CLASSES 1
-#endif
 #endif
 #endif

% cabal new-build -w ghc-7.8.4 --constraint='transformers installed'
Build profile: -w ghc-7.8.4 -O1
In order, the following will be built (use -v for more details):
 - free-5.0.2 (lib) (first run)
Preprocessing library for free-5.0.2..
Building library for free-5.0.2..

src/Control/Monad/Free/Class.hs:44:8:
    Could not find module ‘Control.Monad.Trans.Except’
    It is a member of the hidden package ‘transformers-compat-0.6.1.3’.
    Perhaps you need to add ‘transformers-compat’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

src/Data/Functor/Classes/Compat.hs:30:8:
    Could not find module ‘Data.Functor.Classes’
    It is a member of the hidden package ‘transformers-compat-0.6.1.3’.
    Perhaps you need to add ‘transformers-compat’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

it's not about that CPP, the rest of free is broken too.

[RyanGlScott]

OK. In that case, I have to ask: why was the transformers-compat dependency removed in the first place? It seems like it was legitimately being used in various places.

[phadej]

I'm not the author of #172, ping @vmchale

[RyanGlScott]

Would it suffice to just add:

if !impl(ghc >= 8.0)
  build-depends: transformers-compat >= 0.3 && < 1

Back to the .cabal file? If so, I could do this.

[phadej]

in recursion-schemes I'm a little more conservative,

https://github.com/ekmett/recursion-schemes/pull/46/files#diff-14ead692b66720cacc174e7be4d60386R61

I require that with GHC-7.10 we have at least transformers >= 0.4.2.0 (bundled one), and with older I require `transformers-compat. That's because one could also use older transformers than is bundled with GHC (It's stupid, but better to be explicit about that as we put right bounds in place).

[RyanGlScott]

OK, this is evidently more involved than I thought :)

Do you mind putting up a patch which does the right thing?