Functional

A small functional programming utility module for PowerShell. Read Functional Programming in PowerShell to learn more about how to apply functional programming paradigm to PowerShell, as well as see a more conceptual explanation and examples of using this module.

How does functional programming work in PowerShell?

Processing arrays in pipelines with functions

Functional Programming is largely about applying functions to Lists to obtain different results. PowerShell has good support for this as long as the function only takes a single argument, such as with filter functions and their implicit $_ argument.

# square each number
1..10 | % {$_ * $_}

Functions as first-class objects

Functional Programming languages all support functions as first-class objects, which basically means that the function can be assigned to a variable. PowerShell does not support functions as first-class objects—so how can PowerShell support functional programming?

PowerShell supports something similar to functions, called scriptblocks, which are first-class objects.

# define a scriptblock
$add = {
  Param($a, $b)
  $a + $b
}

You can also obtain a scriptblock from a PowerShell function.

# reference the scriptblock from a function
function add($a, $b) {
  $a + $b
}
$add = $function:add

Or from a string.

# load a script as a scriptblock
$add = [scriptblock](Get-Content add.ps1 -Raw)

You can invoke your scriptblock with read/write access to variables in your current scope using . or with only read access to your current scope using &.

$n = 14
function addToN($a) {
  $n = $a + $n
}

&$function:addToN 3
$n                  # `14`
.$function:addToN 3 
$n                  # `17`

How does this module help with functional programming?

Functional Programming is largely about applying functions to Lists to obtain different results. PowerShell pipelines have amazing support for this, but of the three main Functional Programming functions, PowerShell only has support for two.

Input	Output	Python function	PowerShell function
List, function	List of same length	map	ForEach-Object
List, function	List of smaller length	filter	Where-Object
List, function	Any type	reduce	?

This module introduces five cmdlets for functional programming:

• Reduce-Object, for applying a function to each element of the array and an accumulated value, and returning the acculated value.
• Merge-Object, for recursively merging two objects using a given strategy or a custom strategy.
• Test-All, for testing if all elements are truthy
• Test-Any, for testing if any elements are truthy
• Test-Equality, for recursively testing for deep equality

1..10 | Reduce-Object {$a + $b}
# outputs `55`

Merge-Object @{a = @{b = 1}} @{a = @{c = 2}} -Strategy Fail
# outputs `@{a = @{b = 1; c = 2}}`

Where's the rest of the docs?

This PowerShell module is pure PowerShell (not C#) so there are some limitations with using help, but generally, the best way to learn about the cmdlets in the module is to install the module with Install-Module functional and use the help cmdlet to learn more about each cmdlet.

Feel free to file an issue to request specific documentation improvements (or feature requests, bugs, etc).