.hsc files are not reprocessed when CPP definitions change

[ntc2]

In regular .hs files, changing the result of a #ifdef VAR by adding or removing a -DVAR causes recompilation. However, this is not the case for .hsc files: the .hsc files are not reprocessed when when their #ifdef results change. A simple solution might be to regenerate the FOO.hs from FOO.hsc on each build, and then rebuild FOO.hs when the new version differs from the old version. But this would slow down the common case in order to fix the special case.

I uploaded a simple, concrete example at https://github.com/ntc2/cabal-hsc2hs-bug and have pasted the README below:

Illustrate hsc2hs+cabal bug

Problem

The HSC.hsc file does not get reprocessed when the CPP definitions
change, only when the HSC.hsc file itself is changed. On the other
hand, the regular Haskell HS.hs file does get reprocessed when the CPP
definitions change.

Cabal and GHC versions

$ cabal --version
cabal-install version 1.22.6.0
using version 1.22.2.0 of the Cabal library

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.2

Setup

$ cabal sandbox init

Source files

$ cat bug/Main.hs
module Main where

import HS
import HSC

main = do
  putStrLn $ "hsTest  = " ++ show hsTest
  putStrLn $ "hscTest = " ++ show hscTest

$ cat src/HS.hs
{-# LANGUAGE CPP #-}
module HS where

hsTest :: Bool
#ifdef CPP_FLAG
hsTest = True
#else
hsTest = False
#endif

$ cat src/HSC.hsc
{-# LANGUAGE CPP #-}
module HSC where

hscTest :: Bool
#ifdef CPP_FLAG
hscTest = True
#else
hscTest = False
#endif

$ cat cabal-hsc2hs-bug.cabal
name:                cabal-hsc2hs-bug
version:             0.1.0.0
license:             BSD3
license-file:        LICENSE
author:              Nathan Collins
maintainer:          nathan.collins@gmail.com
build-type:          Simple
cabal-version:       >=1.10

flag cabal_flag
  description:         a Cabal flag
  default:             True
  manual:              True

library
  exposed-modules:     HS, HSC
  other-extensions:    CPP
  build-depends:       base
  hs-source-dirs:      src
  build-tools:         hsc2hs
  default-language:    Haskell2010

  if flag(cabal_flag)
    cpp-options:         -DCPP_FLAG
    cc-options:          -DCPP_FLAG

executable bug
  main-is:             Main.hs
  hs-source-dirs:      bug
  build-depends:       base, cabal-hsc2hs-bug
  default-language:    Haskell2010

Observed, buggy behavior

$ for prefix in "+" "-"; do \
    cabal configure --flags="${prefix}cabal_flag" && \
    cabal build -j1 && \
    cabal copy \
    && .cabal-sandbox/bin/bug; \
  done
[...]
hsTest  = True
hscTest = True
[...]
hsTest  = False
hscTest = True

Notice that hscTest is always True. If you instead configure with
the flag disabled the first time, then hscTest is always False.

Expected behavior

By touching the HSC.hsc file first, we can force a regeneration of
the HSC.hs file:

$ for prefix in "+" "-"; do \
    touch src/HSC.hsc && \
    cabal configure --flags="${prefix}cabal_flag" && \
    cabal build -j1 && \
    cabal copy \
    && .cabal-sandbox/bin/bug; \
  done
[...]
hsTest  = True
hscTest = True
[...]
hsTest  = False
hscTest = False

See file FULL_OUTPUT.md if interested in full output.

[ntc2]

Related: #24

[ntc2]

Another simple solution could be to regenerate all hsc2hs files whenever flags change. This is cheap in the common case. However, this wouldn't solve the more general problem #24.

[NicolasT]

Similarly, when hsc2hs-options are changed, hsc2hs isn't re-invoked either.