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id schema-string-validation
title String Validation and Refinements
category primitives
skillLevel beginner
tags
schema
string
validation
refinements
lessonOrder 28
rule
description
String Validation and Refinements using Schema.
summary Raw strings need constraints: minimum length for passwords, maximum length for tweets, email format, URL format. Without validation, invalid strings propagate through your system causing errors far...

Problem

Raw strings need constraints: minimum length for passwords, maximum length for tweets, email format, URL format. Without validation, invalid strings propagate through your system causing errors far from where the bad data entered.

Solution

import { Schema } from "effect"

// ============================================
// BUILT-IN STRING REFINEMENTS
// ============================================

// Non-empty string
const NonEmpty = Schema.NonEmptyString
// ✅ "hello" → "hello"
// ❌ ""      → ParseError

// Trimmed (no leading/trailing whitespace)  
const Trimmed = Schema.Trimmed
// ✅ "hello"    → "hello"
// ❌ "  hello " → ParseError

// Min/max length
const Username = Schema.String.pipe(
  Schema.minLength(3),
  Schema.maxLength(20)
)

// Exact length
const ZipCode = Schema.String.pipe(Schema.length(5))

// ============================================
// PATTERN MATCHING (REGEX)
// ============================================

const Email = Schema.String.pipe(
  Schema.pattern(/^[^\s@]+@[^\s@]+\.[^\s@]+$/, {
    message: () => "Invalid email format"
  })
)

const PhoneUS = Schema.String.pipe(
  Schema.pattern(/^\d{3}-\d{3}-\d{4}$/, {
    message: () => "Phone must be XXX-XXX-XXXX"
  })
)

const Slug = Schema.String.pipe(
  Schema.pattern(/^[a-z0-9]+(?:-[a-z0-9]+)*$/, {
    message: () => "Slug must be lowercase with hyphens"
  })
)

// ============================================
// COMBINING REFINEMENTS
// ============================================

const Password = Schema.String.pipe(
  Schema.minLength(8, { message: () => "Password must be at least 8 characters" }),
  Schema.maxLength(100),
  Schema.pattern(/[A-Z]/, { message: () => "Must contain uppercase letter" }),
  Schema.pattern(/[a-z]/, { message: () => "Must contain lowercase letter" }),
  Schema.pattern(/[0-9]/, { message: () => "Must contain a number" })
)

// ============================================
// USING IN STRUCTS
// ============================================

const UserRegistration = Schema.Struct({
  username: Username,
  email: Email,
  password: Password,
  phone: Schema.optional(PhoneUS),
})

// Decode and validate
const decode = Schema.decodeUnknownSync(UserRegistration)

// Valid input
const validUser = decode({
  username: "alice123",
  email: "alice@example.com",
  password: "SecurePass1",
})
console.log("✅ Valid:", validUser.username)

// Invalid input - shows specific error
try {
  decode({
    username: "ab",  // Too short
    email: "not-an-email",
    password: "weak",
  })
} catch (error) {
  console.log("❌ Validation failed")
}

Why This Works

Refinement Purpose
minLength/maxLength Constrain string length
length Exact length requirement
pattern Match regular expression
NonEmptyString Reject empty strings
Trimmed Reject strings with whitespace padding
Chaining (.pipe) Combine multiple constraints

When to Use

  • Usernames, passwords, emails
  • Phone numbers, zip codes
  • URLs, slugs, identifiers
  • Any string with format requirements

Related Patterns