Interpolate String Loaded From File - swift

Question

I can't figure out how to load a string from a file and have variables referenced in that string be interpolated.
Let's say a text file at filePath that has these contents:
Hello there, \(name)!
I can load this file into a string with:
let string = String.stringWithContentsOfFile(filePath, encoding: NSUTF8StringEncoding, error: nil)!
In my class, I have loaded a name in: let name = "George"
I'd like this new string to interpolate the \(name) using my constant, so that its value is Hello there, George!. (In reality the text file is a much larger template with lots of strings that need to be swapped in.)
I see String has a convertFromStringInterpolation method but I can't figure out if that's the right way to do this. Does anyone have any ideas?

Answer 1

This cannot be done as you intend, because it goes against type safety at compile time (the compiler cannot check type safety on the variables that you are trying to refer to on the string file).
As a workaround, you can manually define a replacement table, as follows:
// Extend String to conform to the Printable protocol
extension String: Printable
{
public var description: String { return self }
}
var string = "Hello there, [firstName] [lastName]. You are [height]cm tall and [age] years old!"
let firstName = "John"
let lastName = "Appleseed"
let age = 33
let height = 1.74
let tokenTable: [String: Printable] = [
"[firstName]": firstName,
"[lastName]": lastName,
"[age]": age,
"[height]": height]
for (token, value) in tokenTable
{
string = string.stringByReplacingOccurrencesOfString(token, withString: value.description)
}
println(string)
// Prints: "Hello there, John Appleseed. You are 1.74cm tall and 33 years old!"
You can store entities of any type as the values of tokenTable, as long as they conform to the Printable protocol.
To automate things further, you could define the tokenTable constant in a separate Swift file, and auto-generate that file by using a separate script to extract the tokens from your string-containing file.
Note that this approach will probably be quite inefficient with very large string files (but not much more inefficient than reading the whole string into memory on the first place). If that is a problem, consider processing the string file in a buffered way.

Answer 2

There is no built in mechanism for doing this, you will have to create your own.
Here is an example of a VERY rudimentary version:
var values = [
"name": "George"
]
var textFromFile = "Hello there, <name>!"
var parts = split(textFromFile, {$0 == "<" || $0 == ">"}, maxSplit: 10, allowEmptySlices: true)
var output = ""
for index in 0 ..< parts.count {
if index % 2 == 0 {
// If it is even, it is not a variable
output += parts[index]
}
else {
// If it is odd, it is a variable so look it up
if let value = values[parts[index]] {
output += value
}
else {
output += "NOT_FOUND"
}
}
}
println(output) // "Hello there, George!"
Depending on your use case, you will probably have to make this much more robust.