1 files changed, 17 insertions, 2 deletions
diff --git a/Makefile b/Makefile
index 978bf5f..869ee4f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,18 @@
+
+define newline
+
+
+endef
+
+shell-quote = '$(subst $(newline),'\$(newline)',$(subst ','\'',$(1)))'
+
+
+ifdef nixpkgs
+  override nixpkgs := $(call shell-quote,$(nixpkgs))
+else
+  nixpkgs = 'import <stockholm>'
+endif
+
 .PHONY: _default
 _default: ghci
 
@@ -7,7 +22,7 @@ clean:
 
 .PHONY: ghci
 ghci: shell.nix
-	nix-shell --arg nixpkgs 'import <stockholm>' --command 'exec ghci -Wall -fobject-code'
+	nix-shell --arg nixpkgs $(nixpkgs) --command 'exec ghci -Wall -fobject-code'
 
 .PHONY: install
 install:
@@ -15,7 +30,7 @@ install:
 
 .PHONY: result
 result: shell.nix
-	nix-build --arg nixpkgs 'import <stockholm>' ./shell.nix
+	nix-build --arg nixpkgs $(nixpkgs) ./shell.nix
 
 shell.nix: $(wildcard *.cabal)
 	cabal2nix --shell . > $@