Nothing

The nothing data type in Filtrera is a special type that represents the absence of a value. This reference guide provides a detailed specification of the nothing data type, its syntax, and usage for users already familiar with the concept.

The nothing type is used to represent a value that does not exist or is not applicable. It is not compatible with any other type and is typically used in situations where a condition does not match or when no pattern matches the input in pattern matching.

Usage

The nothing type is often used in filters and pattern matching to indicate the absence of a value.

Parameters

nothing can be used to indicate an optional parameter without a default value

param input: text | nothing

from input match
  text |> 'input was a text'

In this example, the returned value is ‘input was a text’ if the input parameter was set. Otherwise nothing is returned.

Filters

In filters, nothing is returned when a condition does not match.

let isEven = (x: number) => x mod 2 == 0
let filterEvenNumbers = (numbers: [number]) =>
  numbers where (x => isEven(x))

let result = filterEvenNumbers([1, 2, 3, 4, 5])

from result

In this example, the filterEvenNumbers function filters out even numbers from the list. For odd numbers, the filter condition does not match, and those values effectively become nothing.

Pattern Matching

In pattern matching, nothing is used when no case matches the input.

let describeValue = (v: text | number) =>
  from v match
    x: text |> $'Text: {x}'
    x: number |> $'Number: {x}'

from describeValue('Hello')
from describeValue(42)
from describeValue(true)  // This will result in nothing

In this example, describeValue returns a string description for text and number types. If a different type is passed, it returns nothing.

Summary

The nothing data type in Filtrera represents the absence of a value and is not compatible with any other type. It is commonly used in filters and pattern matching to indicate when a condition does not match or when no case matches the input. By understanding its usage and handling it appropriately, you can create more robust and error-tolerant Filtrera programs.