Compile empty lists to null instead of nothing

This breaks API for how empty lists are compiled.  They used to stand
for no code whatsoever, and now compile to a `null` identifier.

Rationale: `()` compiling to nothing (as it has since
2b47f4b), is pretty useless, given that
there is already a way to make nothing by writing nothing.

The for loop macro was relying on being able to compile () into nothing,
but the fix was pretty simple.

Resolves anko#32.

src/built-in-macros.ls

@@ -99,6 +99,12 @@ function-type = (type) -> (params, ...rest) ->
   params : params
   body : optionally-implicit-block-statement this, rest
 
+compile-unless-empty-list = (compile, ast) ->
+  if ast.type isnt \list
+    throw Error "Unexpected argument AST; expected list"
+  if ast.values.length then compile ast
+                       else null
+
 contents =
   \+ : n-ary-expr \+
   \- : n-ary-expr \-
@@ -236,9 +242,9 @@ contents =
 
   \for : (init, test, update, ...body) ->
     type : \ForStatement
-    init : @compile init
-    test : @compile test
-    update : @compile update
+    init : compile-unless-empty-list @compile, init
+    test : compile-unless-empty-list @compile, test
+    update : compile-unless-empty-list @compile, update
     body : optionally-implicit-block-statement this, body
 
   \forin : (left, right, ...body) ->

src/compile.ls

@@ -108,7 +108,12 @@ list-to-self-producer = (env, { values }) ->
 
 list-to-estree = (env, { values }:ast, options={}) ->
 
-  return null if values.length is 0
+  if values.length is 0
+    return do
+      type : \Literal
+      value : null
+      raw : \null
+      loc : ast.location
 
   [ head, ...rest ] = values
 

test.ls

@@ -204,7 +204,7 @@ test "variable declaration and assignment" ->
 
 test "empty statement" ->
   esl "()"
-    ..`@equals` ""
+    ..`@equals` "null;"
 
 test "break statement" ->
   esl "(while true (break))"
@@ -564,14 +564,14 @@ test "simple unquoting macro" ->
     ..`@equals` "three();"
 
 test "empty-list-returning macro" ->
-  esl "(macro nothing (lambda () (return '())))
-       (nothing)"
-    ..`@equals` ""
+  esl "(macro shouldbenull (lambda () (return '())))
+       (shouldbenull)"
+    ..`@equals` "null;"
 
 test "empty-list-returning macro (using quasiquote)" ->
-  esl "(macro nothing (lambda () (return `())))
-       (nothing)"
-    ..`@equals` ""
+  esl "(macro shouldbenull (lambda () (return `())))
+       (shouldbenull)"
+    ..`@equals` "null;"
 
 test "nothing-returning macro" ->
   esl "(macro nothing (lambda () (return undefined)))