Add expression functions with context boundaries

internal/lang/parser.go

@@ -471,18 +471,28 @@ func (p *parser) parseFunction() (FunctionDecl, error) {
 	if err != nil {
 		return FunctionDecl{}, err
 	}
+	function := FunctionDecl{
+		Name:           signature.Name,
+		TypeParams:     signature.TypeParams,
+		Params:         signature.Params,
+		ReturnType:     signature.ReturnType,
+		ReturnExplicit: signature.ReturnExplicit,
+	}
+	if p.match(tokenAssign) {
+		expression, err := p.parseExpr(0)
+		if err != nil {
+			return FunctionDecl{}, err
+		}
+		function.ExpressionBody = expression
+		function.InferReturn = !signature.ReturnExplicit
+		return function, nil
+	}
 	body, err := p.parseBlock()
 	if err != nil {
 		return FunctionDecl{}, err
 	}
-
-	return FunctionDecl{
-		Name:       signature.Name,
-		TypeParams: signature.TypeParams,
-		Params:     signature.Params,
-		ReturnType: signature.ReturnType,
-		Body:       body,
-	}, nil
+	function.Body = body
+	return function, nil
 }
 
 func (p *parser) parseFunctionSignature() (FunctionSignature, error) {
@@ -509,7 +519,9 @@ func (p *parser) parseFunctionSignature() (FunctionSignature, error) {
 	}
 
 	returnType := "Unit"
+	returnExplicit := false
 	if p.match(tokenColon) {
+		returnExplicit = true
 		typ, err := p.parseTypeRef()
 		if err != nil {
 			return FunctionSignature{}, err
@@ -518,10 +530,11 @@ func (p *parser) parseFunctionSignature() (FunctionSignature, error) {
 	}
 
 	return FunctionSignature{
-		Name:       name.lexeme,
-		TypeParams: typeParams,
-		Params:     params,
-		ReturnType: returnType,
+		Name:           name.lexeme,
+		TypeParams:     typeParams,
+		Params:         params,
+		ReturnType:     returnType,
+		ReturnExplicit: returnExplicit,
 	}, nil
 }