promql/parse.go
func (p *parser) primaryExpr() Expr {
	switch t := p.next(); {
	case t.typ == ItemNumber:
		f := p.number(t.val)
		return &NumberLiteral{f}

	case t.typ == ItemString:
		return &StringLiteral{p.unquoteString(t.val)}

	case t.typ == ItemLeftBrace:
		// Metric selector without metric name.
		p.backup()
		return p.VectorSelector("")

	case t.typ == ItemIdentifier:
		// Check for function call.
		if p.peek().typ == ItemLeftParen {
			return p.call(t.val)
		}
		fallthrough // Else metric selector.

	case t.typ == ItemMetricIdentifier:
		return p.VectorSelector(t.val)

	case t.typ.isAggregator():
		p.backup()
		return p.aggrExpr()

	default:
		p.errorf("no valid expression found")
	}
	return nil
}
func (p *parser) primaryExpr() Expr {
	switch t := p.next(); {
	case t.typ == ItemNumber:
		f := p.number(t.val)
		return &NumberLiteral{f}

	case t.typ == ItemString:
		return &StringLiteral{p.unquoteString(t.val)}

	case t.typ == ItemLeftBrace:
		// Metric selector without metric name.
		p.backup()
		return p.VectorSelector("")

	case t.typ == ItemIdentifier:
		// Check for function call.
		if p.peek().typ == ItemLeftParen {
			return p.call(t.val)
		}
		fallthrough // Else metric selector.

	case t.typ == ItemMetricIdentifier:
		return p.VectorSelector(t.val)

	case t.typ.isAggregator():
		p.backup()
		return p.aggrExpr()

	default:
		p.errorf("no valid expression found")
	}
	return nil
}
func ( p * parser ) primaryExpr ( ) Expr { switch t := p . next ( ) ; { case t . typ == ItemNumber : f := p . number ( t . val ) return & NumberLiteral { f } case t . typ == ItemString : return & StringLiteral { p . unquoteString ( t . val ) } case t . typ == ItemLeftBrace : // Metric selector without metric name. p . backup ( ) return p . VectorSelector ( " " ) case t . typ == ItemIdentifier : // Check for function call. if p . peek ( ) . typ == ItemLeftParen { return p . call ( t . val ) } fallthrough // Else metric selector. case t . typ == ItemMetricIdentifier : return p . VectorSelector ( t . val ) case t . typ . isAggregator ( ) : p . backup ( ) return p . aggrExpr ( ) default : p . errorf ( " " ) } return nil }
// primaryExpr parses a primary expression.
//
//		<metric_name> | <function_call> | <Vector_aggregation> | <literal>
//
primaryExpr parses a primary expression . <metric_name > | <function_call > | <Vector_aggregation > | <literal >