Powershell getting rid of newlines in function - function

Question

I'm trying to run a function that parses a large string that contains newlines, however whenever I pass this string into a function it gets rid of the new lines and makes it impossible to parse. Am I doing something wrong here?
function parseString([string] $s)
{
$result = $s | Select-String -pattern "foo"
return $result
}
If I type:
$s | Select-String -pattern "foo"
I get the correct result but using
parseString $s
returns the whole string with no newlines. Any suggestions?
EDIT: Hmm after messing around a bit I got rid of the [string] so it's
function parseString($s)
This seems to work, but why?

Answer 1

What is $s? If it is an array of string, then since you are saying that parseString takes in a string, the array of string is converted into a string. If on the other hand $s were a single string, it will work ( as shown below):
function parseString([string] $s)
{
$result = $s | Select-String -pattern "foo"
return $result
}
$s =#'
first line
second line with foo
third line
'#
parseString $s
But if $s=#("first line","secondline with foo","third line"), the array of strings is first converted to a string ( by simply joining each string ) and hence you will lose the newline. If you have got $s, from Get-Content etc. this will be the case.
Note that, most of the times, you won't need to specify the types in Powershell. Be it while assigning variables or in function paramaters.
PS:
If you did
$ofs = "`n"
parseString $s
you will get the expected result in the function with [string].

Answer 2

Removing [string] is probably removing the cast that is happening. You can cast object types to other types this way. For example:
$thing = [int] 42
$thing.GetType().FullName
Output: System.Int32
$thing = [string] $thing
$thing.GetType().FullName
Output: System.String
If your input is a string array, [string] will cast it to a single string, like the -join operator.
To see what is happing, print out $YourVariable.GetType().FullName before calling the function and inside the function (still using [string]).