Source: parser.go in package github.com/jmespath/go-jmespath

package jmespath

import (
	"encoding/json"
	"fmt"
	"strconv"
	"strings"
)

type astNodeType int

go:generate stringer -type astNodeType

const (
	ASTEmpty astNodeType = iota
	ASTComparator
	ASTCurrentNode
	ASTExpRef
	ASTFunctionExpression
	ASTField
	ASTFilterProjection
	ASTFlatten
	ASTIdentity
	ASTIndex
	ASTIndexExpression
	ASTKeyValPair
	ASTLiteral
	ASTMultiSelectHash
	ASTMultiSelectList
	ASTOrExpression
	ASTAndExpression
	ASTNotExpression
	ASTPipe
	ASTProjection
	ASTSubexpression
	ASTSlice
	ASTValueProjection
)

ASTNode represents the abstract syntax tree of a JMESPath expression.

type ASTNode struct {
	nodeType astNodeType
	value    interface{}
	children []ASTNode
}

func (node ASTNode) String() string {
	return node.PrettyPrint(0)
}

PrettyPrint will pretty print the parsed AST. The AST is an implementation detail and this pretty print function is provided as a convenience method to help with debugging. You should not rely on its output as the internal structure of the AST may change at any time.

func (node ASTNode) PrettyPrint(indent int) string {
	spaces := strings.Repeat(" ", indent)
	output := fmt.Sprintf("%s%s {\n", spaces, node.nodeType)
	nextIndent := indent + 2
	if node.value != nil {

Account for things like comparator nodes that are enums with a String() method.

			output += fmt.Sprintf("%svalue: %s\n", strings.Repeat(" ", nextIndent), converted.String())
		} else {
			output += fmt.Sprintf("%svalue: %#v\n", strings.Repeat(" ", nextIndent), node.value)
		}
	}
	lastIndex := len(node.children)
	if lastIndex > 0 {
		output += fmt.Sprintf("%schildren: {\n", strings.Repeat(" ", nextIndent))
		childIndent := nextIndent + 2
		for _, elem := range node.children {
			output += elem.PrettyPrint(childIndent)
		}
	}
	output += fmt.Sprintf("%s}\n", spaces)
	return output
}

var bindingPowers = map[tokType]int{
	tEOF:                0,
	tUnquotedIdentifier: 0,
	tQuotedIdentifier:   0,
	tRbracket:           0,
	tRparen:             0,
	tComma:              0,
	tRbrace:             0,
	tNumber:             0,
	tCurrent:            0,
	tExpref:             0,
	tColon:              0,
	tPipe:               1,
	tOr:                 2,
	tAnd:                3,
	tEQ:                 5,
	tLT:                 5,
	tLTE:                5,
	tGT:                 5,
	tGTE:                5,
	tNE:                 5,
	tFlatten:            9,
	tStar:               20,
	tFilter:             21,
	tDot:                40,
	tNot:                45,
	tLbrace:             50,
	tLbracket:           55,
	tLparen:             60,
}

Parser holds state about the current expression being parsed.

type Parser struct {
	expression string
	tokens     []token
	index      int
}

NewParser creates a new JMESPath parser.

func NewParser() *Parser {
	p := Parser{}
	return &p
}

Parse will compile a JMESPath expression.

func (p *Parser) Parse(expression string) (ASTNode, error) {
	lexer := NewLexer()
	p.expression = expression
	p.index = 0
	tokens, err := lexer.tokenize(expression)
	if err != nil {
		return ASTNode{}, err
	}
	p.tokens = tokens
	parsed, err := p.parseExpression(0)
	if err != nil {
		return ASTNode{}, err
	}
	if p.current() != tEOF {
		return ASTNode{}, p.syntaxError(fmt.Sprintf(
			"Unexpected token at the end of the expression: %s", p.current()))
	}
	return parsed, nil
}

func (p *Parser) parseExpression(bindingPower int) (ASTNode, error) {
	var err error
	leftToken := p.lookaheadToken(0)
	p.advance()
	leftNode, err := p.nud(leftToken)
	if err != nil {
		return ASTNode{}, err
	}
	currentToken := p.current()
	for bindingPower < bindingPowers[currentToken] {
		p.advance()
		leftNode, err = p.led(currentToken, leftNode)
		if err != nil {
			return ASTNode{}, err
		}
		currentToken = p.current()
	}
	return leftNode, nil
}

func (p *Parser) parseIndexExpression() (ASTNode, error) {
	if p.lookahead(0) == tColon || p.lookahead(1) == tColon {
		return p.parseSliceExpression()
	}
	indexStr := p.lookaheadToken(0).value
	parsedInt, err := strconv.Atoi(indexStr)
	if err != nil {
		return ASTNode{}, err
	}
	indexNode := ASTNode{nodeType: ASTIndex, value: parsedInt}
	p.advance()
	if err := p.match(tRbracket); err != nil {
		return ASTNode{}, err
	}
	return indexNode, nil
}

func (p *Parser) parseSliceExpression() (ASTNode, error) {
	parts := []*int{nil, nil, nil}
	index := 0
	current := p.current()
	for current != tRbracket && index < 3 {
		if current == tColon {
			index++
			p.advance()
		} else if current == tNumber {
			parsedInt, err := strconv.Atoi(p.lookaheadToken(0).value)
			if err != nil {
				return ASTNode{}, err
			}
			parts[index] = &parsedInt
			p.advance()
		} else {
			return ASTNode{}, p.syntaxError(
				"Expected tColon or tNumber" + ", received: " + p.current().String())
		}
		current = p.current()
	}
	if err := p.match(tRbracket); err != nil {
		return ASTNode{}, err
	}
	return ASTNode{
		nodeType: ASTSlice,
		value:    parts,
	}, nil
}

func (p *Parser) match(tokenType tokType) error {
	if p.current() == tokenType {
		p.advance()
		return nil
	}
	return p.syntaxError("Expected " + tokenType.String() + ", received: " + p.current().String())
}

func (p *Parser) led(tokenType tokType, node ASTNode) (ASTNode, error) {
	switch tokenType {
	case tDot:
		if p.current() != tStar {
			right, err := p.parseDotRHS(bindingPowers[tDot])
			return ASTNode{
				nodeType: ASTSubexpression,
				children: []ASTNode{node, right},
			}, err
		}
		p.advance()
		right, err := p.parseProjectionRHS(bindingPowers[tDot])
		return ASTNode{
			nodeType: ASTValueProjection,
			children: []ASTNode{node, right},
		}, err
	case tPipe:
		right, err := p.parseExpression(bindingPowers[tPipe])
		return ASTNode{nodeType: ASTPipe, children: []ASTNode{node, right}}, err
	case tOr:
		right, err := p.parseExpression(bindingPowers[tOr])
		return ASTNode{nodeType: ASTOrExpression, children: []ASTNode{node, right}}, err
	case tAnd:
		right, err := p.parseExpression(bindingPowers[tAnd])
		return ASTNode{nodeType: ASTAndExpression, children: []ASTNode{node, right}}, err
	case tLparen:
		name := node.value
		var args []ASTNode
		for p.current() != tRparen {
			expression, err := p.parseExpression(0)
			if err != nil {
				return ASTNode{}, err
			}
			if p.current() == tComma {
				if err := p.match(tComma); err != nil {
					return ASTNode{}, err
				}
			}
			args = append(args, expression)
		}
		if err := p.match(tRparen); err != nil {
			return ASTNode{}, err
		}
		return ASTNode{
			nodeType: ASTFunctionExpression,
			value:    name,
			children: args,
		}, nil
	case tFilter:
		return p.parseFilter(node)
	case tFlatten:
		left := ASTNode{nodeType: ASTFlatten, children: []ASTNode{node}}
		right, err := p.parseProjectionRHS(bindingPowers[tFlatten])
		return ASTNode{
			nodeType: ASTProjection,
			children: []ASTNode{left, right},
		}, err
	case tEQ, tNE, tGT, tGTE, tLT, tLTE:
		right, err := p.parseExpression(bindingPowers[tokenType])
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{
			nodeType: ASTComparator,
			value:    tokenType,
			children: []ASTNode{node, right},
		}, nil
	case tLbracket:
		tokenType := p.current()
		var right ASTNode
		var err error
		if tokenType == tNumber || tokenType == tColon {
			right, err = p.parseIndexExpression()
			if err != nil {
				return ASTNode{}, err
			}
			return p.projectIfSlice(node, right)

Otherwise this is a projection.

		if err := p.match(tStar); err != nil {
			return ASTNode{}, err
		}
		if err := p.match(tRbracket); err != nil {
			return ASTNode{}, err
		}
		right, err = p.parseProjectionRHS(bindingPowers[tStar])
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{
			nodeType: ASTProjection,
			children: []ASTNode{node, right},
		}, nil
	}
	return ASTNode{}, p.syntaxError("Unexpected token: " + tokenType.String())
}

func (p *Parser) nud(token token) (ASTNode, error) {
	switch token.tokenType {
	case tJSONLiteral:
		var parsed interface{}
		err := json.Unmarshal([]byte(token.value), &parsed)
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{nodeType: ASTLiteral, value: parsed}, nil
	case tStringLiteral:
		return ASTNode{nodeType: ASTLiteral, value: token.value}, nil
	case tUnquotedIdentifier:
		return ASTNode{
			nodeType: ASTField,
			value:    token.value,
		}, nil
	case tQuotedIdentifier:
		node := ASTNode{nodeType: ASTField, value: token.value}
		if p.current() == tLparen {
			return ASTNode{}, p.syntaxErrorToken("Can't have quoted identifier as function name.", token)
		}
		return node, nil
	case tStar:
		left := ASTNode{nodeType: ASTIdentity}
		var right ASTNode
		var err error
		if p.current() == tRbracket {
			right = ASTNode{nodeType: ASTIdentity}
		} else {
			right, err = p.parseProjectionRHS(bindingPowers[tStar])
		}
		return ASTNode{nodeType: ASTValueProjection, children: []ASTNode{left, right}}, err
	case tFilter:
		return p.parseFilter(ASTNode{nodeType: ASTIdentity})
	case tLbrace:
		return p.parseMultiSelectHash()
	case tFlatten:
		left := ASTNode{
			nodeType: ASTFlatten,
			children: []ASTNode{{nodeType: ASTIdentity}},
		}
		right, err := p.parseProjectionRHS(bindingPowers[tFlatten])
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{nodeType: ASTProjection, children: []ASTNode{left, right}}, nil
	case tLbracket:

var right ASTNode

		if tokenType == tNumber || tokenType == tColon {
			right, err := p.parseIndexExpression()
			if err != nil {
				return ASTNode{}, nil
			}
			return p.projectIfSlice(ASTNode{nodeType: ASTIdentity}, right)
		} else if tokenType == tStar && p.lookahead(1) == tRbracket {
			p.advance()
			p.advance()
			right, err := p.parseProjectionRHS(bindingPowers[tStar])
			if err != nil {
				return ASTNode{}, err
			}
			return ASTNode{
				nodeType: ASTProjection,
				children: []ASTNode{{nodeType: ASTIdentity}, right},
			}, nil
		} else {
			return p.parseMultiSelectList()
		}
	case tCurrent:
		return ASTNode{nodeType: ASTCurrentNode}, nil
	case tExpref:
		expression, err := p.parseExpression(bindingPowers[tExpref])
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{nodeType: ASTExpRef, children: []ASTNode{expression}}, nil
	case tNot:
		expression, err := p.parseExpression(bindingPowers[tNot])
		if err != nil {
			return ASTNode{}, err
		}
		return ASTNode{nodeType: ASTNotExpression, children: []ASTNode{expression}}, nil
	case tLparen:
		expression, err := p.parseExpression(0)
		if err != nil {
			return ASTNode{}, err
		}
		if err := p.match(tRparen); err != nil {
			return ASTNode{}, err
		}
		return expression, nil
	case tEOF:
		return ASTNode{}, p.syntaxErrorToken("Incomplete expression", token)
	}

	return ASTNode{}, p.syntaxErrorToken("Invalid token: "+token.tokenType.String(), token)
}

func (p *Parser) parseMultiSelectList() (ASTNode, error) {
	var expressions []ASTNode
	for {
		expression, err := p.parseExpression(0)
		if err != nil {
			return ASTNode{}, err
		}
		expressions = append(expressions, expression)
		if p.current() == tRbracket {
			break
		}
		err = p.match(tComma)
		if err != nil {
			return ASTNode{}, err
		}
	}
	err := p.match(tRbracket)
	if err != nil {
		return ASTNode{}, err
	}
	return ASTNode{
		nodeType: ASTMultiSelectList,
		children: expressions,
	}, nil
}

func (p *Parser) parseMultiSelectHash() (ASTNode, error) {
	var children []ASTNode
	for {
		keyToken := p.lookaheadToken(0)
		if err := p.match(tUnquotedIdentifier); err != nil {
			if err := p.match(tQuotedIdentifier); err != nil {
				return ASTNode{}, p.syntaxError("Expected tQuotedIdentifier or tUnquotedIdentifier")
			}
		}
		keyName := keyToken.value
		err := p.match(tColon)
		if err != nil {
			return ASTNode{}, err
		}
		value, err := p.parseExpression(0)
		if err != nil {
			return ASTNode{}, err
		}
		node := ASTNode{
			nodeType: ASTKeyValPair,
			value:    keyName,
			children: []ASTNode{value},
		}
		children = append(children, node)
		if p.current() == tComma {
			err := p.match(tComma)
			if err != nil {
				return ASTNode{}, nil
			}
		} else if p.current() == tRbrace {
			err := p.match(tRbrace)
			if err != nil {
				return ASTNode{}, nil
			}
			break
		}
	}
	return ASTNode{
		nodeType: ASTMultiSelectHash,
		children: children,
	}, nil
}

func (p *Parser) projectIfSlice(left ASTNode, right ASTNode) (ASTNode, error) {
	indexExpr := ASTNode{
		nodeType: ASTIndexExpression,
		children: []ASTNode{left, right},
	}
	if right.nodeType == ASTSlice {
		right, err := p.parseProjectionRHS(bindingPowers[tStar])
		return ASTNode{
			nodeType: ASTProjection,
			children: []ASTNode{indexExpr, right},
		}, err
	}
	return indexExpr, nil
}
func (p *Parser) parseFilter(node ASTNode) (ASTNode, error) {
	var right, condition ASTNode
	var err error
	condition, err = p.parseExpression(0)
	if err != nil {
		return ASTNode{}, err
	}
	if err := p.match(tRbracket); err != nil {
		return ASTNode{}, err
	}
	if p.current() == tFlatten {
		right = ASTNode{nodeType: ASTIdentity}
	} else {
		right, err = p.parseProjectionRHS(bindingPowers[tFilter])
		if err != nil {
			return ASTNode{}, err
		}
	}

	return ASTNode{
		nodeType: ASTFilterProjection,
		children: []ASTNode{node, right, condition},
	}, nil
}

func (p *Parser) parseDotRHS(bindingPower int) (ASTNode, error) {
	lookahead := p.current()
	if tokensOneOf([]tokType{tQuotedIdentifier, tUnquotedIdentifier, tStar}, lookahead) {
		return p.parseExpression(bindingPower)
	} else if lookahead == tLbracket {
		if err := p.match(tLbracket); err != nil {
			return ASTNode{}, err
		}
		return p.parseMultiSelectList()
	} else if lookahead == tLbrace {
		if err := p.match(tLbrace); err != nil {
			return ASTNode{}, err
		}
		return p.parseMultiSelectHash()
	}
	return ASTNode{}, p.syntaxError("Expected identifier, lbracket, or lbrace")
}

func (p *Parser) parseProjectionRHS(bindingPower int) (ASTNode, error) {
	current := p.current()
	if bindingPowers[current] < 10 {
		return ASTNode{nodeType: ASTIdentity}, nil
	} else if current == tLbracket {
		return p.parseExpression(bindingPower)
	} else if current == tFilter {
		return p.parseExpression(bindingPower)
	} else if current == tDot {
		err := p.match(tDot)
		if err != nil {
			return ASTNode{}, err
		}
		return p.parseDotRHS(bindingPower)
	} else {
		return ASTNode{}, p.syntaxError("Error")
	}
}

func (p *Parser) lookahead(number int) tokType {
	return p.lookaheadToken(number).tokenType
}

func (p *Parser) current() tokType {
	return p.lookahead(0)
}

func (p *Parser) lookaheadToken(number int) token {
	return p.tokens[p.index+number]
}

func (p *Parser) advance() {
	p.index++
}

func tokensOneOf(elements []tokType, token tokType) bool {
	for _, elem := range elements {
		if elem == token {
			return true
		}
	}
	return false
}

func (p *Parser) syntaxError(msg string) SyntaxError {
	return SyntaxError{
		msg:        msg,
		Expression: p.expression,
		Offset:     p.lookaheadToken(0).position,
	}
}

Create a SyntaxError based on the provided token. This differs from syntaxError() which creates a SyntaxError based on the current lookahead token.

func (p *Parser) syntaxErrorToken(msg string, t token) SyntaxError {
	return SyntaxError{
		msg:        msg,
		Expression: p.expression,
		Offset:     t.position,
	}