dataprep

Composable, type-driven preprocessing and validation combinator library for Gleam.

dataprep is a combinator toolkit, not a rule catalog.

Built-in and user-defined rules are identical in power.
No domain-specific rules (email, URL, UUID). Write your own or use a dedicated package.
No schema, no DSL, no reflection.
Prep transforms. Validator checks. They do not mix.
Errors are your types, not ours.

Requirements

Gleam 1.15 or later
Erlang/OTP 27 or later

Install

gleam add dataprep

Quick start

import dataprep/prep
import dataprep/validated.{type Validated}
import dataprep/rules

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

pub type Err {
  NameEmpty
  AgeTooYoung
}

pub fn validate_user(name: String, age: Int) -> Validated(User, Err) {
  let clean = prep.trim() |> prep.then(prep.lowercase())
  let check_name = rules.not_empty(NameEmpty)
  let check_age = rules.min_int(0, AgeTooYoung)

  validated.map2(
    User,
    name |> clean |> check_name,
    check_age(age),
  )
}

// validate_user("  Alice ", 25)   -> Valid(User("alice", 25))
// validate_user("", -1)           -> Invalid([NameEmpty, AgeTooYoung])

Modules

Module	Responsibility
`dataprep/prep`	Infallible transformations: `trim`, `lowercase`, `uppercase`, `collapse_space`, `replace`, `default`. Compose with `then` or `sequence`.
`dataprep/validator`	Checks without transformation: `check`, `predicate`, `both`, `all`, `alt`, `guard`, `map_error`, `label`.
`dataprep/validated`	Applicative error accumulation: `map`, `map_error`, `and_then`, `from_result`, `to_result`, `map2`..`map5`.
`dataprep/non_empty_list`	At-least-one guarantee for error lists: `single`, `cons`, `append`, `concat`, `map`, `flat_map`, `to_list`, `from_list`.
`dataprep/rules`	Built-in rules: `not_empty`, `min_length`, `max_length`, `min_int`, `max_int`, `one_of`, `equals`.

Composition overview

Phase	Combinator	Errors	When to use
Prep	`prep.then`	(none)	Chain infallible transforms
Validate	`validator.both` / `all`	Accumulate all	Independent checks on same value
Validate	`validator.alt`	Accumulate on full failure	Accept alternative forms
Validate	`validator.guard`	Short-circuit	Skip if prerequisite fails
Combine	`validated.map2`..`map5`	Accumulate all	Build domain types from independent fields
Bridge	`validated.and_then`	Short-circuit	Parse then validate (type changes)
Bridge	`raw \|> prep \|> validator`	(prep has none)	Apply infallible transform before validation

Development

This project uses mise to manage Gleam and Erlang versions, and just as a task runner.

mise install    # install Gleam and Erlang
just ci         # format check, typecheck, build, test
just test       # gleam test
just format     # gleam format
just check      # all checks without deps download

Contributing

Contributions are welcome. See CONTRIBUTING.md for details.

License

MIT