From 3cd79689f40b8accb4befe40e6edb7a8b12c9b39 Mon Sep 17 00:00:00 2001
From: Will Fancher <elvishjerricco@gmail.com>
Date: Tue, 8 Jan 2019 16:40:12 -0500
Subject: [PATCH] Limit parallel building of Haskell packages. [Fixes #53665]

---
 pkgs/development/haskell-modules/generic-builder.nix | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pkgs/development/haskell-modules/generic-builder.nix b/pkgs/development/haskell-modules/generic-builder.nix
index fc2c008be0c8..77bde5c85b72 100644
--- a/pkgs/development/haskell-modules/generic-builder.nix
+++ b/pkgs/development/haskell-modules/generic-builder.nix
@@ -78,6 +78,7 @@ in
   # same package in the (recursive) dependencies of the package being
   # built. Will delay failures, if any, to compile time.
   allowInconsistentDependencies ? false
+, maxBuildCores ? 4 # GHC usually suffers beyond -j4. https://ghc.haskell.org/trac/ghc/ticket/9221
 } @ args:
 
 assert editedCabalFile != null -> revision != null;
@@ -250,6 +251,7 @@ stdenv.mkDerivation ({
   '' + postPatch;
 
   setupCompilerEnvironmentPhase = ''
+    NIX_BUILD_CORES=$(( NIX_BUILD_CORES < ${toString maxBuildCores} ? NIX_BUILD_CORES : ${toString maxBuildCores} ))
     runHook preSetupCompilerEnvironment
 
     echo "Build with ${ghc}."