Jump out of function recursive function
I wrote a parser in Objective-C about a very ambiguous language . Everything works fine. I often jump out of many function calls by throwing an exception (I continue to be in my code, so the exception will never exit the parser).
I was thinking about porting the code to Swift, then I noticed that there is no exception handling in Swift.
I don't need exception handling, and I don't need it , but an upstart from many different nested functions was called when using parser writing .
Is there a good way instead of passing exceptions to a block @catch
in Swift?
An example of a recursive function to make it clearer (playground working code):
import Foundation
func addToSum(sum: Int, rest: String) -> Int {
if rest == "" {
return sum
} else {
if rest[rest.endIndex.predecessor()] != "i" {
// can't throw exception, if scratches is like "iiiiiiiiiiiioiiiiii"
}
var shorterRest = rest.substringToIndex(rest.endIndex.predecessor())
return addToSum(sum + 1, shorterRest)
}
}
let scratches = "iiiiiiiiiiiiiiiiiiiii"
var sum = addToSum(0, scratches) // sum is 11 ;-)
I know the example is not that good because it is only one function and you can easily add an error, but then I had to go through each function call.
See a larger picture. I don't need any reverse processing in the parser.
Here is a link that has the same problem: use exceptions as flow control in parsers. Unfortunately, he doesn't answer.
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In this case, it is fairly easy to handle by returning an option. Instead of throwing an exception, just return nil
and let it propagate the chain fallback.
func addToSum(sum: Int, rest: String) -> Int? {
if rest == "" {
return sum
} else {
if rest[rest.endIndex.predecessor()] != "i" {
return nil
}
var shorterRest = rest.substringToIndex(rest.endIndex.predecessor())
return addToSum(sum + 1, shorterRest)
}
}
let scratches = "iiiiiiiiiiiiiiiiiiiii"
var sum = addToSum(0, scratches) // sum is 21, actually
sum = addToSum(0, "iiiioiii") // sum is now nil
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