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Hegel is a property-based testing library for Go. Hegel is based on Hypothesis, using the Hegel protocol.

To install: go get hegel.dev/go/hegel@latest.

Hegel will use uv to install the required hegel-core server component. If uv is already on your path, it will use that, otherwise it will download a private copy of it to ~/.cache/hegel and not put it on your path. See https://hegel.dev/reference/installation for details.

Here's a quick example of how to write a Hegel test:

package example_test

import (
	"math"
	"slices"
	"testing"

	"hegel.dev/go/hegel"
)

func mySort(ls []int) []int {
	result := make([]int, len(ls))
	copy(result, ls)
	slices.Sort(result)
	result = slices.Compact(result)
	return result
}

func TestMatchesBuiltin(t *testing.T) {
	t.Run("matches builtin", hegel.Case(func(ht *hegel.T) {
		slice1 := hegel.Draw(ht, hegel.Lists(hegel.Integers(math.MinInt, math.MaxInt)))
		slice2 := mySort(slice1)
		slices.Sort(slice1)
		if !slices.Equal(slice1, slice2) {
			ht.Fatalf("slices not equal: %v != %v", slice1, slice2)
		}
	}))
}

This test will fail when run with go test! Hegel will produce a minimal failing test case for us:

Draw 1: [0 0]
    example_test.go:25: slices not equal: [0 0] != [0]

Hegel reports the minimal example showing that our sort is incorrectly dropping duplicates. If we remove result = slices.Compact(result) from mySort(), this test will then pass (because it's just comparing the standard sort against itself).