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CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

What is Fender

Fender is a Go validation library that provides a unified, composable way to validate data. It uses generics to build type-safe validation pipelines with three layers: fender (orchestrator), fend (field-level validators), and rule (individual validation rules).

Commands

All commands go through make. Run make help for the full list.

  • make check — full pipeline: tidy, generate, lint, test
  • make lint / make lint.fix — run golangci-lint (config in .golangci.yml)
  • make test — run tests with -tags=safe
  • make test.race — run tests with race detector
  • make test.update — run tests with -update flag
  • Single test: go test -tags=safe -v -run TestName ./path/to/package/...
  • Tool setup: make .mise (installs mise dependencies), make .lefthook (configures git hooks)

Build tag -tags=safe is required for all builds and tests.

Architecture

Three-layer validation pipeline, each layer wrapping the one below:

rule/ — Atomic validation rules. A rule.Rule[T] is func(ctx context.Context, v T) error. Returns *rule.Error on validation failure or rule.ErrBreak to short-circuit. Includes built-in rules: RequiredString, MinString, Email, Match, Range, UUID, etc. Type aliases (StringRule, IntRule, etc.) provide convenience.

fend/ — Field-level validators. A fend.Fend is func(ctx context.Context, mode Mode) error. Created via fend.Field(name, value, rules...), fend.Var(value, rules...), or fend.DynamicField/fend.DynamicVar for runtime-typed rules. fend.Union implements OR-logic across rule sets. fend.Rules[T] allows prepending shared rules to fields.

fender.go (root) — Orchestrator with three validation modes:

  • fender.All — runs all fends, collects all errors from all rules
  • fender.First — stops at first fend with an error, returns only its first rule error
  • fender.AllFirst — runs all fends but returns only the first fend's errors

config/ — Shared configuration: a go-playground/validator instance and delimiter/regex constants.

Error hierarchy: fender.Error contains []*fend.Error, each containing []*rule.Error. Use fender.AsError(), fend.AsError(), rule.AsError() to unwrap. String format uses ; between rule errors and : between fend errors.