# Validator
A validation library for [Gleam](https://gleam.run/).
This library follows the principle [Parse don't validate](https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/).
You start with an input type and validate into an output type. These two types can be different. For example:
```haskell
type UserInput { UserInput(name: Option(String), age: Int) }
type ValidUser { ValidUser(name: String, age: Int) }
```
Then you create a validator like:
```haskell
import validator
import validator/common.{ValidatorResult}
import validator/int
import validator/option
fn user_validator(user: UserInput) -> ValidatorResult(ValidUser, String) {
validator.build2(ValidUser)
|> validator.validate(user.name, option.is_some("Please provide a name"))
|> validator.validate(user.age, int.min(13, "Must be at least 13 years old"))
}
```
And run it:
```rust
case user_validator(input) {
Ok(valid_user) -> ...
Error(tuple(first_error, all_errors)) -> ...
}
```
## Error type
Errors can be your own type e.g.
```haskell
import validator
import validator/common.{ValidatorResult}
import validator/int
import validator/option
type Error {
ErrorEmptyName,
ErrorTooYoung,
}
fn user_validator(user: UserInput) -> ValidatorResult(ValidUser, String) {
validator.build2(ValidUser)
|> validator.validate(user.name, option.is_some(ErrorEmptyName))
|> validator.validate(user.age, int.min(13, ErrorTooYoung))
}
```
## ValidatorResult
`ValidatorResult(valid, error)` is an alias for `Result(valid, tuple(error, List(error)))`
The `Ok` branch has the valid output.
The `Error` branch has a tuple `tuple(error, List(error))`.
The first value is the first error. The second value is a list with all errors (including the first).
## Custom property validator
A property validator has two components:
- The error to return
- A function that transforms the property if successful (`fn(input) -> Option(output)`)
Example:
```rust
fn bigger_than_10(num: Int) -> Option(num) {
case num > 10 {
True ->
Some(num)
False ->
None
}
}
let custom = validator.custom("Must be bigger than 10", bigger_than_10)
let validator = fn(form: FormInput) {
validator.build1(ValidForm)
|> validator.validate(form.quantity, custom)
}
```
## Test
```sh
rebar3 compile
rebar3 eunit
```
## Installation
This package can be installed by adding `gleam_validator` to your `rebar.config` dependencies:
```erlang
{deps, [
gleam_validator
]}.
```
