Some more work on string expansion.

README.md

@@ -11,42 +11,44 @@ of it. Put simply, mk takes make, keeps its simple direct syntax, but fixes
 basically everything that's annoyed you over the years. To name a few things:
 
   1. Recipes are delimited by any indentation, not tab characters in particular.
-  2. Phony targets are handled separately from file targets. Your mkfile won't
+  1. Phony targets are handled separately from file targets. Your mkfile won't
      be broken by having a file named 'clean'.
-  2. Attributes instead of weird special targets like `.SECONDARY:`.
-  5. Special variables like `$target`, `$prereq`, and `$stem` in place of
+  1. Attributes instead of weird special targets like `.SECONDARY:`.
+  1. Special variables like `$target`, `$prereq`, and `$stem` in place of
      make's pointlessly cryptic `$@`, `$^`, and `$*`.
-  3. In addition to suffix rules (e.g. `%.o: %.c`), mk has more powerful regular
+  1. In addition to suffix rules (e.g. `%.o: %.c`), mk has more powerful regular
      expression rules.
-  4. Sane handling of rules with multiple targets.
-  5. An optional attribute to delete targets when a recipe fails, so you aren't
+  1. Sane handling of rules with multiple targets.
+  1. An optional attribute to delete targets when a recipe fails, so you aren't
      left with corrupt output.
-  6. Plan 9 mkfiles can not only include other mkfiles, but pipe in the output of
+  1. Plan 9 mkfiles can not only include other mkfiles, but pipe in the output of
      recipes. Your mkfile can configure itself by doing something like
      `<|sh config.sh`.
-  7. A generalized mechanism to determine if a target is out of date, for when
+  1. A generalized mechanism to determine if a target is out of date, for when
      timestamps won't cut it.
+  1. Variables are expanded in recipes only if they are defined. They way you
+     usually don't have to escape `$`.
 
 And much more! For more, read the original mk paper: ["Mk: a successor to
 make"](#).
 
 # Improvements over Plan 9 mk
 
-This mk stays mostly faithful to Plan 9, but makes a few minor (in my opinion)
+This mk stays mostly faithful to Plan 9, but makes a few (in my opinion)
 improvements.
 
+  1. A clean, modern implementation in go, that doesn't depend on the whole plan
+     9 for userspace stack.
+  1. Use go regular expressions, which are perl-like. The original mk used plan9
+     regex, which few people know or care to learn.
   1. Allow blank lines in recipes. A recipe is any indented block of text, and
      continues until a non-indented character or the end of the file.
-  2. Add an 'S' attribute to execute recipes with programs other than sh. This
+  1. Add an 'S' attribute to execute recipes with programs other than sh. This
      way, you don't have to separate your six line python script into its own
      file. Just stick it in the mkfile.
-  3. Use a perl-compatible regular expressions. The original mk used plan9
-     regex, which few people know or care to learn.
-  4. A clean, modern implementation in go, that doesn't depend on the whole plan
-     9 for userspace stack.
+  1. Use sh syntax for command insertion (i.e. backticks) rather than rc shell
+     syntax.
 
-Most Plan 9 mkfiles should remain backwards compatible, but strict backwards
-compatibility isn't the goal.
 
 # Current State