Add while statement and allow recursion (#39)

Adds a flag, AllowRecursion, that enables while statements and
recursive functions.

4 files changed, 33 insertions, 2 deletions

diff --git a/syntax/grammar.txt b/syntax/grammar.txt
index 8c90272..c3ad159 100644
--- a/syntax/grammar.txt
+++ b/syntax/grammar.txt
@@ -4,7 +4,7 @@ Grammar of Starlark
 
 File = {Statement | newline} eof .
 
-Statement = DefStmt | IfStmt | ForStmt | SimpleStmt .
+Statement = DefStmt | IfStmt | ForStmt | WhileStmt | SimpleStmt .
 
 DefStmt = 'def' identifier '(' [Parameters [',']] ')' ':' Suite .
 
@@ -16,6 +16,8 @@ IfStmt = 'if' Test ':' Suite {'elif' Test ':' Suite} ['else' ':' Suite] .
 
 ForStmt = 'for' LoopVariables 'in' Expression ':' Suite .
 
+WhileStmt = 'while' Test ':' Suite .
+
 Suite = [newline indent {Statement} outdent] | SimpleStmt .
 
 SimpleStmt = SmallStmt {';' SmallStmt} [';'] '\n' .
diff --git a/syntax/parse.go b/syntax/parse.go
index 6bc12b3..08a40e3 100644
--- a/syntax/parse.go
+++ b/syntax/parse.go
@@ -111,6 +111,8 @@ func (p *parser) parseStmt(stmts []Stmt) []Stmt {
 		return append(stmts, p.parseIfStmt())
 	} else if p.tok == FOR {
 		return append(stmts, p.parseForStmt())
+	} else if p.tok == WHILE {
+		return append(stmts, p.parseWhileStmt())
 	}
 	return p.parseSimpleStmt(stmts)
 }
@@ -182,6 +184,18 @@ func (p *parser) parseForStmt() Stmt {
 	}
 }
 
+func (p *parser) parseWhileStmt() Stmt {
+	whilepos := p.nextToken() // consume WHILE
+	cond := p.parseTest()
+	p.consume(COLON)
+	body := p.parseSuite()
+	return &WhileStmt{
+		While: whilepos,
+		Cond:  cond,
+		Body:  body,
+	}
+}
+
 // Equivalent to 'exprlist' production in Python grammar.
 //
 // loop_variables = primary_with_suffix (COMMA primary_with_suffix)* COMMA?
diff --git a/syntax/scan.go b/syntax/scan.go
index 37bd38e..695a120 100644
--- a/syntax/scan.go
+++ b/syntax/scan.go
@@ -95,6 +95,7 @@ const (
 	OR
 	PASS
 	RETURN
+	WHILE
 
 	maxToken
 )
@@ -996,6 +997,7 @@ var keywordToken = map[string]Token{
 	"or":       OR,
 	"pass":     PASS,
 	"return":   RETURN,
+	"while":    WHILE,
 
 	// reserved words:
 	"as": ILLEGAL,
@@ -1011,7 +1013,6 @@ var keywordToken = map[string]Token{
 	"nonlocal": ILLEGAL,
 	"raise":    ILLEGAL,
 	"try":      ILLEGAL,
-	"while":    ILLEGAL,
 	"with":     ILLEGAL,
 	"yield":    ILLEGAL,
 }
diff --git a/syntax/syntax.go b/syntax/syntax.go
index 29b22de..c4f64c0 100644
--- a/syntax/syntax.go
+++ b/syntax/syntax.go
@@ -95,6 +95,7 @@ func (*BranchStmt) stmt() {}
 func (*DefStmt) stmt()    {}
 func (*ExprStmt) stmt()   {}
 func (*ForStmt) stmt()    {}
+func (*WhileStmt) stmt()  {}
 func (*IfStmt) stmt()     {}
 func (*LoadStmt) stmt()   {}
 func (*ReturnStmt) stmt() {}
@@ -351,6 +352,19 @@ func (x *ForStmt) Span() (start, end Position) {
 	return x.For, end
 }
 
+// A WhileStmt represents a while loop: while X: Body.
+type WhileStmt struct {
+	commentsRef
+	While Position
+	Cond  Expr
+	Body  []Stmt
+}
+
+func (x *WhileStmt) Span() (start, end Position) {
+	_, end = x.Body[len(x.Body)-1].Span()
+	return x.While, end
+}
+
 // A ForClause represents a for clause in a list comprehension: for Vars in X.
 type ForClause struct {
 	commentsRef