Network.URI - MIN_VERSION_base problem, or finger-trouble?

[Daniel Fischer]

On Samstag, 29. Dezember 2012, 18:25:45, Graham Klyne wrote:
> A couple of problems with Network.URI have been brought to my attention, so
> I thought I'd have a go at seeing if I could fix them.  It's been a while
> (years) since I used the Haskell toolchain in anger, so I'm a bit out of
> touch with how thinks are working now.
> 
> So far, I have:
> 
> 1. Installed Haskell platform (64-bit version for MacOS).
> 2. Checked out the 'network' project from https://github.com/haskell/network
> 3. cabal install test-framework
> 4. cabal install test-framework-hunit
> 5. changed to the 'test' directory of the checked out 'network' project
> 
> Then when I try to run GHC, I get this error:
<snip>
> 
> The problem seems to be with the following lines in URI.hs (starting at line
> 135): [[
> # if MIN_VERSION_base(4,0,0)
> import Data.Data (Data)
> # else
> import Data.Generics (Data)
> # endif
> ]]
> 
> If I remove these lines, and just leave
> [[
> import Data.Generics (Data)
> ]]
> 
> I can compile and run the test suite just fine:
> [[
>           Test Cases    Total
>   Passed  319           319
>   Failed  0             0
>   Total   319           319
> ]]
> 
> I'm _guessing_ the problem here is something to do with
> "MIN_VERSION_base(4,0,0)" - not being familiar with the current build
> environment setup, I'm not sure where it is or should be defined, or what it
> is intended to mean.

The MIN_VERSION_package is a macro that cabal creates when building the 
package, that checks whether the available package version satisfies the 
minimum requirements.

When hacking on a package without using cabal, you can

a) remove the problematic macro
b) define it in the file yourself
c) pass -D"MIN_VERSION_base(x,y,z)=1" on the command line

(I've tested option c only for C code, it might not work here, but afaik, 
gcc's preprocessor is used, so I think it will).

Since you will probably compile more than once, a) or b) would be the 
preferable options.

But you could also leave the macro as is and just

$ cabal build

the package. That needs one

$ cabal configure

before the first build, after that, a `cabal build` will only recompile 
changed modules (and what depends on them), so the compilation should be 
reasonably fast.

> 
> I did find these:
>   * http://www.haskell.org/pipermail/haskell-cafe/2012-August/102787.html
>   * http://stackoverflow.com/questions/8961413/zlib-build-error-with-ghc
> but they didn't help me.
> 
> It seems there's a fix that involves using __GLASGOW_HASKELL__ instead of
> MIN_VERSION_base(4,0,0), but I'm not sure exactly how that would play out -
> or indeed if that's the correct approach for a permanent fix.

In principle, the MIN_VERSION macro is the right thing, because it tests for 
the package version where things changed. For base, a suitable 
__GLASGOW_HASKELL__ condition is of course equivalent to a base-version check.