Schema validation for Gleam — constraints, error accumulation, and field paths.

All errors are collected in a single pass using the use pattern. No short-circuiting, no missing fields.

gleam add sift@1

import sift
import sift/string as s
import sift/int as i

pub type UserInput {
  UserInput(name: String, email: String, age: Int)
}

pub type User {
  User(name: String, email: String, age: Int)
}

pub fn validate(input: UserInput) -> Result(User, List(sift.FieldError)) {
  use name <- sift.check("name", input.name, s.min_length(1, "required"))
  use email <- sift.check("email", input.email, s.email("invalid email"))
  use age <- sift.check("age", input.age, i.between(0, 150, "out of range"))
  sift.ok(User(name:, email:, age:))
  |> sift.validate
}

Invalid input returns all errors at once:

validate(UserInput(name: "", email: "nope", age: -1))
// -> Error([
//   FieldError(path: ["name"], message: "required"),
//   FieldError(path: ["email"], message: "invalid email"),
//   FieldError(path: ["age"], message: "out of range"),
// ])

For a full walkthrough — nested structs, optional fields, conditional validation with when, multiple validators per field with check_all, and per-item list validation with each — see example/contacts/.

• Error accumulation — every field is checked, every error is returned
• Field paths — nested structs and lists produce paths like ["address", "zip"] or ["tags", "0"]
• Multi-validator — sift.check_all runs multiple validators on one field, collects all errors
• Composable — sift.and, sift.or, sift.not to combine validators
• Conditional — sift.when runs a validator only when a condition is true
• Nested structs — sift.nested with automatic path prefixing
• List validation — sift.each and sift.check_each with indexed paths
• Cross-field — sift.check2 in-chain, sift.refine post-assembly
• Optional fields — sift.check_optional, sift/option.required, sift/option.optional
• Form parsing — sift.check_parse bridges raw strings to typed values
• Built-in validators — strings, ints, floats, lists, options

Further documentation can be found at https://hexdocs.pm/sift.