diff --git a/tools/makefile b/tools/makefile
index 21c2f0b..4ab64d8 100644
--- a/tools/makefile
+++ b/tools/makefile
@@ -1,4 +1,6 @@
 ## dirs
+dir_lib := lib
+dir_source := source
 dir_temp := temp
 dir_build := build
 
@@ -10,13 +12,13 @@ cmd_chmod := chmod
 cmd_typescript_compile := tsc --removeComments --lib es2018
 
 ## rules
-build/type2: source/head.js lib/plankton/plankton.js temp/unlinked.js
+${dir_build}/type2: ${dir_source}/head.js ${dir_lib}/plankton/plankton.js ${dir_temp}/unlinked.js
 	@ ${cmd_log} "linking …"
 	@ ${cmd_directory_create} ${dir_build}
 	@ ${cmd_concatenate} $^ > $@
 	@ ${cmd_chmod} +x $@
 
-temp/unlinked.js: lib/plankton/plankton.d.ts source/base.ts source/type2.ts
+${dir_temp}/unlinked.js: ${dir_lib}/plankton/plankton.d.ts ${dir_source}/base.ts ${dir_source}/type2.ts
 	@ ${cmd_log} "compiling …"
 	@ ${cmd_directory_create} ${dir_temp}
 	@ ${cmd_typescript_compile} $^ --outFile $@