Powershell's -split behaves differently depending on method parameters

If you split a string, you get a string[].

$foo = "apple,banana,coconut"
Write-Output $foo.GetType()

$foo = $foo -split ","
Write-Output $foo.GetType()

If you split a paramaterized string, you get a string.

Function Get-Foo() {
    param ( 
      [string] $foo
    )

    Write-Output $foo.GetType()

    $foo = $foo -split ","
    Write-Output $foo.GetType()
}

Why?

I researched this and didn't turn anything up; Powershell's documentation on split was largely unhelpful.

The difference is that in your first example, you never declared the variable, so you can assign it values of any type and it will accept them without coercion, altering its type to match:

PS > $foo = "apple,banana,coconut"
PS > Write-Output $foo.GetType().Name
String
PS > $foo = $foo -split ","
PS > Write-Output $foo.GetType().Name
String[]
PS > $foo = 1
PS > Write-Output $foo.GetType().Name
Int32

But once you've explicitly declared $foo's type with [string], $foo.GetType() will continue to report string, and anything you assign to it will be coerced to string. You can't change the type of a declared variable just by assigning it a new value:

PS > [string] $foo = "apple,banana,coconut"
PS > Write-Output $foo.GetType().Name
String
PS > $foo = $foo -split ","
PS > Write-Output $foo.GetType().Name
String
PS > $foo = 1
PS > Write-Output $foo.GetType().Name
String

You can change the type by re-declaring it, though. Even within a function:

PS > [Int32] $foo = $foo
PS > Write-Output $foo.GetType().Name
Int32

Note that such a redeclaration assignment will fail if the value is not interpretable as the new type:

PS > [string] $foo = "apple,banana,coconut"
PS > [Int32] $foo = $foo
Cannot convert value "apple,banana,coconut" to type "System.Int32". Error: "Input string was not in a correct format."

As far as I know, there's no way to "un-declare" a variable's type so it goes back to accepting any type of value, short of removing the variable entirely with Remove-Variable.