Evaluator

The evaluator package implements a small expression language for querying Go structs. Expressions are represented as Go structs that can be combined using logical operators. Comparison expressions support both numeric and string values.

Features

• Equality and inequality checks (Is, IsNot)
• Numeric and lexical comparisons (GT, GTE, LT, LTE)
• Membership checks with Contains
• Logical composition using And, Or and Not
• JSON serialisation for easy storage or transmission of queries

Basic Usage

Create a query using the provided expression types and call Evaluate with your target struct:

q := evaluator.Query{
    Expression: &evaluator.AndExpression{Expressions: []evaluator.Query{
        {Expression: &evaluator.IsExpression{Field: "Name", Value: "bob"}},
        {Expression: &evaluator.GreaterThanExpression{Field: "Age", Value: 30}},
    }},
}

matched := q.Evaluate(&User{Name: "bob", Age: 35})

JSON Queries

Queries can be marshalled to and from JSON. This is handy for configuration files or network APIs.

js := `{
  "Expression": {
    "Type": "Contains",
    "Expression": {
      "Field": "Tags",
      "Value": "go"
    }
  }
}`
var q evaluator.Query
if err := json.Unmarshal([]byte(js), &q); err != nil {
    log.Fatal(err)
}

Expression Guide

Each query expression implements the Expression interface. The table below lists the available types and their purpose:

Type	Purpose
Is / IsNot	Check equality or inequality of a field
GT / GTE	Numeric or lexical "greater than" comparisons
LT / LTE	Numeric or lexical "less than" comparisons
Contains	Test that a slice field contains a value
And / Or / Not	Compose other expressions logically

Example usage:

evaluator.Query{Expression: &evaluator.NotExpression{Expression: evaluator.Query{
    Expression: &evaluator.IsExpression{Field: "Deleted", Value: true},
}}}

Command-line Tools

The project includes small utilities for working with common data formats.

• csvfilter – filters CSV rows. Usage: csvfilter -e '<expression>' [file ...] When no files are specified, input is read from standard input.
• jsonlfilter – filters newline-delimited JSON records. It accepts the same arguments as csvfilter and writes matching JSON objects to standard output.
• jsontest – evaluates a single JSON document. It exits with status 0 if the expression matches and 1 otherwise. Multiple files can be supplied and all must satisfy the expression. With no files it reads from standard input.
• yamltest – like jsontest but for YAML documents.

Expressions use the simple syntax implemented by the parser in parser/simple. For example:

jsonlfilter -e 'GT(age,30) and Is(country,"US")' users.jsonl

Running Tests

Run go test ./... to execute the unit tests.

License

This project is licensed under the MIT License.