predicate

README

Predicate

Predicate package used to create interpreted mini languages with Go syntax - mostly to define various predicates for configuration, e.g.

Latency() > 40 || ErrorRate() > 0.5.

Here's an example of fully functional predicate language to deal with division remainders:

package main

import (
  "log"
	
  "github.com/vulcand/predicate"
)

// takes number and returns true or false
type numberPredicate func(v int) bool

// Converts one number to another
type numberMapper func(v int) int

// Function that creates predicate to test if the remainder is 0
func divisibleBy(divisor int) numberPredicate {
     return func(v int) bool {
         return v%divisor == 0
     }
}

// Function - logical operator AND that combines predicates
func numberAND(a, b numberPredicate) numberPredicate {
    return func(v int) bool {
        return a(v) && b(v)
    }
}

func main(){
    // Create a new parser and define the supported operators and methods
    p, err := predicate.NewParser(predicate.Def{
        Operators: predicate.Operators{
           AND: numberAND,
        },
        Functions: map[string]interface{}{
            "DivisibleBy": divisibleBy,
        },
    })

    pr, err := p.Parse("DivisibleBy(2) && DivisibleBy(3)")
    if err != nil {
		log.Fatalf("Error: %v", err)
	}

	fmt.Println(pr.(numberPredicate)(2)) // false
	fmt.Println(pr.(numberPredicate)(3)) // false
	fmt.Println(pr.(numberPredicate)(6)) // true
}

Documentation

Overview

Package predicate used to create interpreted mini languages with Go syntax - mostly to define various predicates for configuration, e.g. Latency() > 40 || ErrorRate() > 0.5.

Here's an example of fully functional predicate language to deal with division remainders:

    // takes number and returns true or false
    type numberPredicate func(v int) bool

    // Converts one number to another
    type numberMapper func(v int) int

    // Function that creates predicate to test if the remainder is 0
    func divisibleBy(divisor int) numberPredicate {
	    return func(v int) bool {
		    return v%divisor == 0
        }
    }

    // Function - logical operator AND that combines predicates
    func numberAND(a, b numberPredicate) numberPredicate {
        return func(v int) bool {
            return a(v) && b(v)
        }
    }

    p, err := NewParser(Def{
		Operators: Operators{
			AND: numberAND,
		},
		Functions: map[string]interface{}{
			"DivisibleBy": divisibleBy,
		},
	})

	pr, err := p.Parse("DivisibleBy(2) && DivisibleBy(3)")
    if err == nil {
        fmt.Fatalf("Error: %v", err)
    }
    pr.(numberPredicate)(2) // false
    pr.(numberPredicate)(3) // false
    pr.(numberPredicate)(6) // true

Index

• func GetFieldByTag(ival interface{}, tagName string, fieldNames []string) (interface{}, error)
• func GetStringMapValue(mapVal, keyVal interface{}) (interface{}, error)
• type BoolPredicate
  • func And(a, b BoolPredicate) BoolPredicate
  • func Contains(a interface{}, b interface{}) BoolPredicate
  • func Equals(a interface{}, b interface{}) BoolPredicate
  • func Not(a BoolPredicate) BoolPredicate
  • func Or(a, b BoolPredicate) BoolPredicate
• type Def
• type GetIdentifierFn
• type GetPropertyFn
• type Operators
• type Parser
  • func NewParser(d Def) (Parser, error)

Functions

func GetFieldByTag

func GetFieldByTag(ival interface{}, tagName string, fieldNames []string) (interface{}, error)

GetFieldByTag returns a field from the object based on the tag.

func GetStringMapValue

func GetStringMapValue(mapVal, keyVal interface{}) (interface{}, error)

GetStringMapValue is a helper function that returns property from map[string]string or map[string][]string the function returns empty value in case if key not found In case if map is nil, returns empty value as well.

Types

type BoolPredicate

type BoolPredicate func() bool

BoolPredicate is a function without arguments that returns boolean value when called.

func And

func And(a, b BoolPredicate) BoolPredicate

And is a boolean predicate that calls two boolean predicates and returns result of && operation on their return values.

func Contains

func Contains(a interface{}, b interface{}) BoolPredicate

Contains checks if string slice contains a string Contains([]string{"a", "b"}, "b") -> true.

func Equals

func Equals(a interface{}, b interface{}) BoolPredicate

Equals can compare complex objects, e.g. arrays of strings and strings together.

func Not

func Not(a BoolPredicate) BoolPredicate

Not is a boolean predicate that calls a boolean predicate and returns negated result.

func Or

func Or(a, b BoolPredicate) BoolPredicate

Or is a boolean predicate that calls two boolean predicates and returns result of || operation on their return values.

type Def

type Def struct {
	Operators Operators
	// Function matching is case sensitive, e.g. Len is different from len
	Functions map[string]interface{}
	// GetIdentifier returns value of any identifier passed in the form []string{"id", "field", "subfield"}
	GetIdentifier GetIdentifierFn
	// GetProperty returns property from a map
	GetProperty GetPropertyFn
}

Def contains supported operators (e.g. LT, GT) and functions passed in as a map.

type GetIdentifierFn

type GetIdentifierFn func(selector []string) (interface{}, error)

GetIdentifierFn function returns identifier based on selector e.g. id.field.subfield will be passed as. GetIdentifierFn([]string{"id", "field", "subfield"}).

type GetPropertyFn

type GetPropertyFn func(mapVal, keyVal interface{}) (interface{}, error)

GetPropertyFn returns property from a mapVal by key keyVal.

type Operators

type Operators struct {
	EQ  interface{}
	NEQ interface{}

	LT interface{}
	GT interface{}

	LE interface{}
	GE interface{}

	OR  interface{}
	AND interface{}
	NOT interface{}
}

Operators contain functions for equality and logical comparison.

type Parser

type Parser interface {
	Parse(in string) (interface{}, error)
}

Parser takes the string with expression and calls the operators and functions.

func NewParser

func NewParser(d Def) (Parser, error)