bug#48944: haskell build-system cannot build pcg-random

[John Kehayias]

Hello,

I also see this error for building packages that need cabal-doctest, as the 
configure step needs the package database. There's actually some packages in 
guix that already modify the configure step to do this, like ghc-cairo: 
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/haskell-xyz.scm#n1465

As noted there, the package database should be passed to runhaskell itself, not 
just what it runs. This would allow other packages that need a module for 
configuring to work. Attached is a patch to modify haskell-build-system to do 
this, though not only for the configure command (seemed easiest to just do that 
for now). I'm submitting the patch with comments there about possible 
modifications.

I can confirm with this that pcg-random builds, as well as packages I've been 
trying that need cabal-doctest (note, I tested using guix's package of 
cabal-doctest, no need to add that as well).

Comments welcome here, especially as I'm pretty new to Haskell still and guix 
completely, or on patch submission (sending shortly).

John

runhaskell-add-pkgdb.patch
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