Monadic Validation in Gleam!

gleam add validate_monadic

import validate_monadic as validate

Usage

This module is the minimal set of functions around a single type alias for Result so it may be used as a “validation monad”.

pub type Validation(validated, error) =
 Result(validated, ErrorList(error))

pub type ErrorList(error) =
 #(error, List(error))

More convenient abstractions can be built on top of this.

Check out the tests for examples around usage:

pub fn small_form_test() {
 let form = Form(first_name: "Tony", last_name: "Bradley", age: "33")

 let validate_form =
   function.curry3(fn(first_name, last_name, age) {
     ValidatedForm(first_name, last_name, age)
   })

 let first_name_result =
   form.first_name
   |> string_non_empty
   |> validate.and_then(string_shorter_than(_, 100))
   |> validate.map(FirstName)
   |> validate.map_error(string.append("First Name Error: ", _))

 let last_name_result =
   form.last_name
   |> string_non_empty
   |> validate.and_then(string_shorter_than(_, 100))
   |> validate.map(LastName)
   |> validate.map_error(string.append("Last Name Error: ", _))

 let age_result =
   form.age
   |> is_postive_int
   |> validate.map(Age)
   |> validate.map_error(string.append("Age Error: ", _))

 let validation_result =
   validate.succeed(validate_form)
   |> validate.and_map(first_name_result)
   |> validate.and_map(last_name_result)
   |> validate.and_map(age_result)

 should.be_ok(validation_result)
}

Development

gleam run   # Run the project
gleam test  # Run the tests
gleam shell # Run an Erlang shell