PowershellSpecs.ps1

before_each { 
  write-host "I will be written before each test is run"
}

when "True is true" {
  assert { $true -eq $true } 
}

when "True is false, the world may explode" {
  assert { $true -eq $false } "True should be false"
}

PsSpec.ps1

# Usage: PsSpec.ps1 path/to/specs.ps1

param(
  $specfile = $(throw "specfile required")
)

$script:specs = @()
$script:beforeeaches = @()

function assert([scriptblock]$assertion, $message = "") {
  $result = & $assertion
  if(-not $result) {
     throw $message
  }
}

function before_each([scriptblock]$block) { 
  $script:beforeeaches += $block
}

function when($condition, [scriptblock]$spec) {
    $specMeta = @{
      "name" = $condition;
      "spec" = $spec;
    }
    
    $script:specs += $specMeta
}

. (resolve-path $specfile)

$specs | % { 
  $beforeeaches | % {  & $_ }
  $meta = $_
  try {
    & $_.spec
    write-host $meta.name -foregroundcolor green
  } catch {
    write-host $meta.name -foregroundcolor red
  }
}