### Define and Handle HTML Forms in Gleam

Source: https://hexdocs.pm/formal/index

Demonstrates defining a Gleam form schema for a `SignUp` type with email and password fields. It includes validation for email format, minimum password length, and password confirmation. The `handle_form_submission` function shows how to process incoming form data, decode it into the defined type, and handle success or error outcomes.

```gleam
import formal/form

// Define a type that is to be decoded from the form data
pub type SignUp {
  SignUp(email: String, password: String)
}

/// A form that decodes the `Signup` value.
fn signup_form() -> Form(Signup) {
  form.new({
    use email <- form.field("email", {
      form.parse_email
    })
    use password <- form.field("password", {
      form.parse_string
      |> form.check_string_length_more_than(7)
    })
    use _ <- form.field("confirm", {
      form.parse_string
      |> form.check_confirms(password)
    })
    SignUp(email: email, password: password)
  })
}

// This function takes the list of key-value string pairs that a HTML form
// produces. It then decodes the form data into a SignUp value, ensuring that
// all the fields are present and valid.
//
pub fn handle_form_submission(values: List(#(String, String))) {
  let result = 
    signup_form()
    |> form.add_values(values)
    |> form.run

  case result {
    Ok(data) -> {
      // Do something with the SignUp value here
    }
    Error(form) -> {
      // Re-render the form with the error messages
    }
  }
}

```

--------------------------------

### Gleam: new function to create a form

Source: https://hexdocs.pm/formal/formal/form

The `new` function creates a new form instance from a given schema. Users can add values using `add_*` functions and then execute the form with `run` to obtain the final value or any encountered errors.

```Gleam
pub fn new(schema: Schema(model)) -> Form(model)
```

```Gleam
let schema = {
  use name <- form.field("name", {
    form.parse_string
    |> form.check_not_empty
  })
  use name <- form.field("age", form.parse_int)
  form.success(Person(name:, age:))
}
let form = form.new(schema)
```

--------------------------------

### Run Form (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Run a form, returning either the successfully parsed value if there are no errors, or a new instance of the form with the errors added to the fields.

```gleam
pub fn run(form: Form(model)) -> Result(model, Form(model))
```

--------------------------------

### Parse URL (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser for URLs. Uses the `gleam/uri` module to parse and validate URLs. Returns a `MustBeUrl` error if the input cannot be parsed as a valid URI.

```gleam
pub const parse_url: Parser(uri.Uri)
```

--------------------------------

### Form Field Handling (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Functions for managing form fields within the Formal library. `field` adds a parser for a named form field, `field_value` retrieves the first string value for a field, and `field_values` retrieves all string values for a field.

```gleam
pub fn field(
  name: String,
  parser: Parser(value),
  continuation: fn(value) -> Schema(model),
) -> Schema(model)
# Add a new parser to the form for a given form field name.
```

```gleam
pub fn field_value(form: Form(model), name: String) -> String
# Get the first values for a given form field.

let form = form |> form.add_int("one", 100)
assert form.field_value(form, "one") == "100"
assert form.field_value(form, "two") == ""
```

```gleam
pub fn field_values(
  form: Form(model),
  name: String,
) -> List(String)
# Get all the values for a given form field.

let form = form
  |> form.add_int("one", 100)
  |> form.add_string("one", "Hello")
  |> form.add_string("two", "Hi!")
assert form.field_values(form, "one") == ["Hello", "100"]
```

--------------------------------

### all_values Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Fetches all the key-value pairs currently stored within the form. This provides a comprehensive view of all data associated with the form.

```gleam
pub fn all_values(form: Form(model)) -> List(#(String, String))
```

--------------------------------

### Gleam: parse function for custom input parsing

Source: https://hexdocs.pm/formal/formal/form

The `parse` function enables the creation of custom parsers for any data type. It requires a function that takes a list of strings and returns a `Result`. If parsing fails, it must provide a default 'zero' value and an error message string.

```Gleam
pub fn parse(
  parser: fn(List(String)) -> Result(t, #(t, String)),
) -> Parser(t)
```

```Gleam
form.parse(fn(input) {
  case input {
    ["Squirtle", ..] -> Ok(Squirtle)
    ["Bulbasaur", ..] -> Ok(Bulbasaur)
    ["Charmander", ..] -> Ok(Charmander)
    _ -> Error(#(Squirtle, "must be a starter Pokémon"))
  }
})
```

--------------------------------

### Parse Optional (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser that applies another parser if there is a non-empty-string input value for the field.

```gleam
pub fn parse_optional(
  parser: Parser(output),
) -> Parser(option.Option(output))
```

--------------------------------

### Parse Float (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser for floating point numbers. Returns a `MustBeFloat` error if the input cannot be parsed as a float.

```gleam
pub const parse_float: Parser(Float)
```

--------------------------------

### Parse String (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Parse a string value. This parser can never fail.

```gleam
pub const parse_string: Parser(String)
```

--------------------------------

### Gleam: map function for data transformation

Source: https://hexdocs.pm/formal/formal/form

The `map` function transforms the parsed value of a parser, similar to `list.map` or `result.map`. It takes a parser and a mapper function, returning a new parser that applies the mapper to the successfully parsed value.

```Gleam
pub fn map(
  parser: Parser(t1),
  mapper: fn(t1) -> t2,
) -> Parser(t2)
```

--------------------------------

### Parse Time (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser for time values. Parses times in HH:MM:SS or HH:MM format and returns a `calendar.TimeOfDay` value. Returns a `MustBeTime` error if the input cannot be parsed as a valid time.

```gleam
pub const parse_time: Parser(calendar.TimeOfDay)
```

--------------------------------

### Error Translation Functions (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Functions to translate `FieldError` types into user-friendly strings. `en_gb` provides British English translations, while `en_us` provides American English translations, notably for the word 'color'.

```gleam
pub fn en_gb(error: FieldError) -> String
# Translates `FieldError`s into strings suitable for showing to the user.

assert en_us(MustBeColour) == "must be a hex colour code"
```

```gleam
pub fn en_us(error: FieldError) -> String
# Translates `FieldError`s into strings suitable for showing to the user.
# The same as `en_gb`, but with Americanised spelling of the word “color”.

assert en_us(MustBeColour) == "must be a hex color code"
```

--------------------------------

### Parse List (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser that applies another parser to each input value in a list. Takes a parser for a single value and returns a parser that can handle multiple values of the same type. This is useful for form fields that can have multiple values, such as checkboxes, multi-selects, or just repeated inputs of other types.

```gleam
pub fn parse_list(parser: Parser(output)) -> Parser(List(output))
```

--------------------------------

### Gleam: parse_checkbox parser for boolean input

Source: https://hexdocs.pm/formal/formal/form

The `parse_checkbox` is a parser for boolean values from checkbox inputs. An absent value is `False`, while any present value (including an empty string or 'on') is `True`. This is useful for handling user agreements or options.

```Gleam
pub const parse_checkbox: Parser(Bool)
```

```Gleam
let schema = {
  use agreed <- form.field("terms-and-conditions", {
    form.parse_checkbox
    |> form.check_accepted
  })
  form.success(Signup(agreed:))
}
```

--------------------------------

### Parse Int (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser for a whole number. Returns a `MustBeInt` error if the input cannot be parsed as a valid int.

```gleam
pub const parse_int: Parser(Int)
```

--------------------------------

### Gleam: language function for form translation

Source: https://hexdocs.pm/formal/formal/form

The `language` function in the Formal library allows supplying a translation function for `FieldError`s. This function converts errors into user-friendly text. It supports built-in languages like `en_gb` and `en_us`, defaulting to `en_gb` if no language is specified.

```Gleam
pub fn language(
  form: Form(model),
  translator: fn(FieldError) -> String,
) -> Form(model)
```

--------------------------------

### formal/form: Form Type

Source: https://hexdocs.pm/formal/formal/form

An opaque type representing a form, created from a Schema using the 'new' function. It is used for supplying values via 'add_*' functions and processing with 'run'.

```APIDOC
Form(model): opaque
  A form! Created from a `Schema` with the `new` function.

  Supply values to a form with the `add_*` functions and then pass it to the
  `run` function to get either the resulting value or any errors.

  Use the `language` function to supply a new translation function to change
  the language of the error messages returned by the `field_error_text` function.
  The default language is `en_gb` English.
```

--------------------------------

### Check Integer Less Than Limit (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Ensures that an integer value parsed by a `Parser` is less than a specified limit. This is useful for validating input ranges.

```gleam
pub fn check_int_less_than(
  parser: Parser(Int),
  limit: Int,
) -> Parser(Int)
```

```gleam
let schema = {
  use age <- form.field("age", {
    form.parse_int
    |> form.check_int_less_than(150)
  })
  form.success(Person(age:))
}
```

--------------------------------

### add_values Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Adds multiple key-value pairs to a form. This function is ideal for populating a form with data originating from sources like HTTP request form bodies or HTML form submissions.

```gleam
pub fn add_values(
  form: Form(a),
  values: List(#(String, String)),
) -> Form(a)
```

```gleam
use formdata <- wisp.require_form(request)
let form <- new_user_form() |> form.add_values(formdata.values)
```

--------------------------------

### formal/form: FieldError Type and Constructors

Source: https://hexdocs.pm/formal/formal/form

Defines the possible errors that can occur during form validation. Includes specific error types like MustBeInt, MustBeEmail, and length/range checks, as well as a CustomError option.

```APIDOC
FieldError:
  MustBePresent
  MustBeInt
  MustBeFloat
  MustBeEmail
  MustBePhoneNumber
  MustBeUrl
  MustBeDate
  MustBeTime
  MustBeDateTime
  MustBeColour
  MustBeStringLengthMoreThan(limit: Int)
  MustBeStringLengthLessThan(limit: Int)
  MustBeIntMoreThan(limit: Int)
  MustBeIntLessThan(limit: Int)
  MustBeFloatMoreThan(limit: Float)
  MustBeFloatLessThan(limit: Float)
  MustBeAccepted
  MustConfirm
    For confirmation of passwords, etc. Must match the first field.
  MustBeUnique
    For values that must be unique. e.g. user email addresses.
  CustomError(message: String)
```

--------------------------------

### Parse Email (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser that validates email addresses. Performs basic email validation by checking for the presence of an “@” symbol. Returns a `MustBeEmail` error if the input is not a valid email format.

```gleam
pub const parse_email: Parser(String)
```

--------------------------------

### formal/form: Schema Type

Source: https://hexdocs.pm/formal/formal/form

An opaque type representing a schema, which is a description of how to decode typed data from form data. Schemas are used to create new form objects via the 'new' function.

```APIDOC
Schema(model): opaque
  A description of how to decode from typed value from form data. This can be
  used to create a new form object using the `new` function.
```

--------------------------------

### Gleam: parse_colour parser for hex color strings

Source: https://hexdocs.pm/formal/formal/form

The `parse_colour` parser accepts color values in HTML hex format (e.g., '#FF0000'). It returns the hex color string upon successful parsing. Invalid formats will result in a `MustBeColour` error.

```Gleam
pub const parse_colour: Parser(String)
```

```Gleam
let schema = {
  use background_color <- form.field("background_color", form.parse_colour)
  form.success(Theme(background_color:))
}
```

--------------------------------

### Check String Length Less Than Limit (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Ensures that a string value parsed by a `Parser` has a length less than a specified limit. Useful for input field constraints.

```gleam
pub fn check_string_length_less_than(
  parser: Parser(String),
  limit: Int,
) -> Parser(String)
```

```gleam
let schema = {
  use username <- form.field("username", {
    form.parse_string
    |> form.check_string_length_less_than(20)
  })
  form.success(User(username:))
}
```

--------------------------------

### Check Integer More Than Limit (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Ensures that an integer value parsed by a `Parser` is more than a specified limit. This is useful for validating input ranges.

```gleam
pub fn check_int_more_than(
  parser: Parser(Int),
  limit: Int,
) -> Parser(Int)
```

```gleam
let schema = {
  use age <- form.field("age", {
    form.parse_int
    |> form.check_int_more_than(0)
  })
  form.success(Person(age:))
}
```

--------------------------------

### Check String Not Empty (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Ensures that a string value parsed by a `Parser` is not an empty string. This is commonly used for required text fields.

```gleam
pub fn check_not_empty(parser: Parser(String)) -> Parser(String)
```

```gleam
let schema = {
  use tags <- form.field("tag", {
    form.parse_string
    |> form.check_not_empty
  })
  form.success(Article(tags:))
}
```

--------------------------------

### Parse Phone Number (Gleam)

Source: https://hexdocs.pm/formal/formal/form

A parser for phone numbers. Phone numbers are checked with rules for length, digits, and optional leading '+', allowing '-', ' ', '(', and ')' characters which are removed. Returns a `MustBePhoneNumber` error if the input doesn’t satisfy these rules.

```gleam
pub const parse_phone_number: Parser(String)
```

--------------------------------

### Gleam: parse_date_time parser for datetime values

Source: https://hexdocs.pm/formal/formal/form

The `parse_date_time` parser handles datetime strings in the HTML 'datetime-local' format (e.g., '2023-12-25T14:30'). It returns a tuple containing `calendar.Date` and `calendar.TimeOfDay`. Invalid formats will trigger a `MustBeDateTime` error.

```Gleam
pub const parse_date_time: Parser(
  #(calendar.Date, calendar.TimeOfDay),
)
```

```Gleam
let schema = {
  use created_at <- form.field("created_at", form.parse_date_time)
  form.success(Event(created_at:))
}
```

--------------------------------

### Gleam: parse_date parser for YYYY-MM-DD dates

Source: https://hexdocs.pm/formal/formal/form

The `parse_date` parser is designed to parse dates in the 'YYYY-MM-DD' format. It returns a `calendar.Date` value. If the input string does not conform to this format, a `MustBeDate` error is returned.

```Gleam
pub const parse_date: Parser(calendar.Date)
```

```Gleam
let schema = {
  use birth_date <- form.field("birth_date", form.parse_date)
  form.success(Person(birth_date:))
}
```

--------------------------------

### Check String Length More Than Limit (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Ensures that a string value parsed by a `Parser` has a length more than a specified limit. Useful for input field constraints like password length.

```gleam
pub fn check_string_length_more_than(
  parser: Parser(String),
  limit: Int,
) -> Parser(String)
```

```gleam
let schema = {
  use password <- form.field("password", {
    form.parse_string
    |> form.check_string_length_more_than(8)
  })
  form.success(User(password:))
}
```

--------------------------------

### all_errors Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Retrieves all errors present within a form. If no validation has been run or errors added via `add_error`, this function will return an empty list.

```gleam
pub fn all_errors(
  form: Form(model),
) -> List(#(String, List(FieldError)))
```

--------------------------------

### Form Error Retrieval (Gleam)

Source: https://hexdocs.pm/formal/formal/form

Functions to inspect errors associated with a form. `field_error_messages` retrieves a list of user-friendly error messages for a specific field, using the configured translator. `field_errors` retrieves the raw `FieldError` types for a field.

```gleam
pub fn field_error_messages(
  form: Form(model),
  name: String,
) -> List(String)
# Get the error messages for a field, if there are any.
# The text is formatted using the translater function given with the
# `langauge` function. The default translater is `en_gb`.
```

```gleam
pub fn field_errors(
  form: Form(model),
  name: String,
) -> List(FieldError)
# Get all the form.
# If the `run` function or the `add_error` function have not been called then
# the form is clean and won’t have any errors yet.
```

--------------------------------

### Set Form Values in Gleam

Source: https://hexdocs.pm/formal/formal/form

Replaces any existing values of a form with new values. This function is useful for adding values from a HTTP request form body sent to your server, or from a HTML form element in your browser-based application.

```gleam
pub fn set_values(
  form: Form(a),
  values: List(#(String, String)),
) -> Form(a)
```

```gleam
use formdata <- wisp.require_form(request)
let form <- new_user_form() |> form.set_values(formdata.values)
```

--------------------------------

### check_accepted Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Ensures that a boolean value parsed by the parser is `True`. This is commonly used for checkboxes that must be explicitly accepted, like terms and conditions.

```gleam
pub fn check_accepted(parser: Parser(Bool)) -> Parser(Bool)
```

```gleam
let schema = {
  use discount <- form.field("terms-and-conditions", {
    form.parse_checkbox
    |> form.check_accepted
  })
  form.success(Product(discount:))
}
```

--------------------------------

### formal/form: Parser Type and Usage

Source: https://hexdocs.pm/formal/formal/form

An opaque type representing a parser, used to extract a value from form data, convert it to a desired type, and optionally validate it. Parsers are used with the 'field' function and exhibit short-circuiting behavior.

```APIDOC
Parser(value): opaque
  A parser extracts a value from from values, converting it to a desired type
  and optionally validating the value. Parsers are used with the `field`
  function.

  See the `parse_*` and `check_*` functions for more information.

  Functions that start with `parse_*` are *short-circuiting*, so any parser
  functions that come afterwrads will not be run. For example, given this
  code:

  ```gleam
  form.parse_int |> form.check_int_more_than(0)
  ```

  If the input is not an int then `parse_int` will fail, causing
  `check_int_more_than` not to run, so the errors will be `[MustBeInt]`.
```

--------------------------------

### check Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Applies a custom validation check to a parser. This allows for complex validation rules beyond basic type parsing, returning either the validated value or a string error message.

```gleam
pub fn check(
  parser: Parser(b),
  checker: fn(b) -> Result(b, String),
) -> Parser(b)
```

```gleam
let must_not_be_rude = fn(text) {
  case contains_swear_word(text) {
    True -> Error("must not be rude")
    False -> Ok(text)
  }
}
let schema = {
  use name <- form.field("name", {
    form.parse_string
    |> form.check(must_not_be_rude)
  })
  form.success(Profile(name:))
}
```

--------------------------------

### check_float_less_than Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Validates that a float value is strictly less than a specified limit. This is useful for setting upper bounds on numerical inputs.

```gleam
pub fn check_float_less_than(
  parser: Parser(Float),
  limit: Float,
) -> Parser(Float)
```

```gleam
let schema = {
  use discount <- form.field("discount", {
    form.parse_float
    |> form.check_float_less_than(100.0)
  })
  form.success(Product(discount:))
}
```

--------------------------------

### add_string Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Adds a string value to a designated field within the form. This is commonly used to pre-populate form fields with data that the user can then modify.

```gleam
pub fn add_string(
  form: Form(model),
  field: String,
  value: String,
) -> Form(model)
```

--------------------------------

### check_float_more_than Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Ensures that a float value is strictly greater than a specified limit. This is commonly used to enforce minimum values for numerical inputs.

```gleam
pub fn check_float_more_than(
  parser: Parser(Float),
  limit: Float,
) -> Parser(Float)
```

```gleam
let schema = {
  use price <- form.field("price", {
    form.parse_float
    |> form.check_float_more_than(0.0)
  })
  form.success(Product(price:))
}
```

--------------------------------

### check_confirms Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Validates that a field's value matches another specified value. This is frequently used for password confirmation fields to ensure consistency.

```gleam
pub fn check_confirms(parser: Parser(t), other: t) -> Parser(t)
```

```gleam
let schema = {
  use password <- form.field("password-confirmation", {
    form.parse_string
    |> form.check_string_length_more_than(8)
  })
  use _ <- form.field("password-confirmation", {
    form.parse_string
    |> form.check_confirms(password)
  })
  form.success(User(password:))
}
```

--------------------------------

### Finalize Parser in Gleam

Source: https://hexdocs.pm/formal/formal/form

Finalises a parser, having successfully parsed a value. This function is used to mark the successful completion of a parsing operation.

```gleam
pub fn success(value: model) -> Schema(model)
```

--------------------------------

### add_int Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Adds an integer value to a specified field in the form. This function is particularly helpful for pre-filling form fields with existing data, allowing users to edit previously saved values.

```gleam
pub fn add_int(
  form: Form(model),
  field: String,
  value: Int,
) -> Form(model)
```

--------------------------------

### add_error Function - Gleam

Source: https://hexdocs.pm/formal/formal/form

Adds a specific error to a field within a form. This is useful for integrating custom validation logic that runs outside the standard form schema, allowing error messages to be displayed to the user through the form interface.

```gleam
pub fn add_error(
  form: Form(model),
  name: String,
  error: FieldError,
) -> Form(model)
```

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