[Nix-dev] Generated path names within a derivation

[Waisbrot, Nathaniel J. (Cont, ARL/CISD)]

> 2) To solve your problem you can concatenate the path with a string.
> Unfortunately, I am not sure this will recompile the package if its
> patch changes.
> 
> patches = [ (./ + "/${version}.patch") ];


  patches = [ (./. + "/${version}.patch") ];

is a more concise form of what I was doing before.  Unfortunately, it
does not pull the file into the store, and therefore does not recompile
when the patch changes.



> 3) If the solution (2) does not recompile when the content of the
> patch file change, then I'll suggest you to generate for each patch a
> small nix expression which contains the relative path to the patch:


OK, this answers my question then.  Any files that I want Nix to copy
into the store automagically must be named directly, not computed.

I've got a kludge-around that appears to do what I want:

  iFile = builtins.readFile "/nix/nixpkgs/foo/${version}.patch";
  oFile = builtins.toFile "${version}.patch" iFile;
  patches = [ oFile ];


1) Nix reads the computed patch file's contents into a variable
2) Nix write the contents of that variable back into the store
3) Nix references the file in the store as a patchfile

Nix notices updates here, I assume because I'm able to force all
variables to be evaluated before we reach the builder.

Thanks for the help!

-- 
Nathaniel Waisbrot
Software Dev Analyst
Lockheed Martin / ARL DoD Supercomputing Resource Center
Adelphi Laboratory Center, MD 20783
Phone:  301-394-0096
Fax:    301-394-2279
Email:  nathaniel.waisbrot at arl.army.mil
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