[Nix-dev] replaceStdenv question

[Marc Weber]

On Sun, Jun 15, 2008 at 12:27:45AM +0200, Peter Simons wrote:
> Hi,
> 
> I observe some weird behavior with following ~/.nixpkg/config.nix file:
> 
>   {
>     replaceStdenv = { stdenv, overrideSetup }: stdenv //
>     {
>       mkDerivation = args: stdenv.mkDerivation (args // { configureFlags = "CFLAGS=-O3"; });
>     };
>   }
> 
> When I install pcre from the unstable packages using "nix-env -i pcre",
> the call to configure is exactly as I'd it expect to be:
> 
>   configure flags: --disable-dependency-tracking                        \
>     --prefix=/nix/store/aaxgzf9qs1m2jwvmnfi88lk97n2ya9nn-pcre-7.4       \
>     CFLAGS=-O3
> 
> When, I add the following attribute to config.nix
> 
>   {
>     replaceStdenv = ... ;
> 
>     pcre = { unicode = true; };
>   }
> 
> ..., however, my custom CFLAGS disappear:
> 
>   configure flags: --disable-dependency-tracking                        \
>     --prefix=/nix/store/dx1xhm7z0wip21wiaxfig8zrsqywan85-pcre-7.4       \
>     --enable-unicode-properties --enable-shared --disable-static
> 
> I would assume that my overridden mkDerivation function evaluates
> *after* the pcre builder, so as far as I can tell, configureFlags should
> have the value my functions sets. But obviously that is not the case.
> Why is that?
> 
> Any insight would be appreciated.
I can't give you insight right now, but maybe a tip how to debug it:
There is builtins.trace, so you can write your own trace fuctnion

traceCF = location : args : builtins.trace 
        "${location}: ${ if args ?  configureFlags then "flags are: ${args.configureFlags}" else "no flags"}"
        args
m
Then you can use it this way and hope that it will show you what you need ?
>       mkDerivation = args: stdenv.mkDerivation (traceCF "before passing to mkDerivation" (args // { configureFlags = "CFLAGS=-O3"; }) );

also use something such as 
postUnpack = "export"; to see which env vars has been passed to the
builder.

pcre contains:
  configureFlags =
    (if unicodeSupport then
      "--enable-unicode-properties --enable-shared --disable-static"
    else "") +
    (if !cplusplusSupport then "--disable-cpp" else "");

I really wonder what will happen if you enable unicodeSupport and do not
disable cplusplusSupport.. doesn't this result in
        "--enable-unicode-properties --enable-shared --disable-static--disable-cpp" ?
                                                                     ^^ no  space

Marc Weber