nw-pbt-go

PBT Go -- rapid, gopter

Framework Selection

Framework	Shrinking	Stateful	API Style	Choose When
rapid	Internal (Hypothesis-like)	Yes (StateMachine)	Idiomatic Go (*rapid.T)	Default choice. Simpler API, better shrinking.
gopter	Type-based	Yes (commands)	Explicit generator construction	Need maximum control over generation

Default: rapid. More idiomatic Go API and fully automatic shrinking.

Quick Start (rapid)

import (
    "sort"
    "testing"
    "pgregory.net/rapid"
)

func TestSortPreservesLength(t *testing.T) {
    rapid.Check(t, func(t *rapid.T) {
        xs := rapid.SliceOf(rapid.Int()).Draw(t, "xs")
        sorted := make([]int, len(xs))
        copy(sorted, xs)
        sort.Ints(sorted)
        if len(sorted) != len(xs) {
            t.Fatalf("length changed: %d -> %d", len(xs), len(sorted))
        }
    })
}

// Run: go test

Generator Cheat Sheet (rapid)

Primitives

rapid.Int()                           // any int
rapid.IntRange(0, 99)                 // bounded
rapid.Int32()
rapid.Float64()
rapid.String()
rapid.StringN(1, 50, -1)             // min 1, max 50 chars
rapid.Bool()
rapid.Byte()

Collections

rapid.SliceOf(rapid.Int())
rapid.SliceOfN(rapid.Int(), 1, 10)   // min 1, max 10 elements
rapid.MapOf(rapid.String(), rapid.Int())

Combinators

rapid.OneOf(rapid.Int(), rapid.Int32()) // does not work for different types
rapid.SampledFrom([]string{"a", "b", "c"})
rapid.Just(42)

// Map (transform)
rapid.Map(rapid.Int(), func(x int) int { return x * 2 }) // even integers

// Filter
rapid.Filter(rapid.Int(), func(x int) bool { return x > 0 })
// Prefer: rapid.IntRange(1, math.MaxInt)

// Custom generator (draw pattern)
func userGen(t *rapid.T) User {
    return User{
        Name: rapid.StringN(1, 20, -1).Draw(t, "name"),
        Age:  rapid.IntRange(1, 120).Draw(t, "age"),
    }
}
// Use: rapid.Custom(userGen)