Get line causing error in catch?

Using Swift
1

Hi
So if I have something that goes like this

import Somelib
do {
    try someFunction()
    ...
catch {
    print(error)
}

Asuming I have more than call to one function with an error, Is there a way I can also get the line causing the error?
Note, Not using XCode here.

2

You mean the line that did the actual throw error, or the whole "stack of lines" like so:

foo.swift:123
bar.swift:456
someFunction.swift:789
someLib.swift:111 // the line where `try someFunction()` is

To do that automatically swift would have to modify the "error" being returned (e.g. add a userValue dictionary key with, file/line info). Swift doesn't do this.

For you own app you can throw NSError and include the relevant file/line info as a userInfo key.

func makeError(... file: String = __FILE__, line: Int = __LINE__) -> Error {
    let userInfo = ["errorFile": file, "errorLine": line]
    let error = NSError(..., userInfo: userInfo)
    return error
}

...
if ... {
    throw makeError(...) // this file + line would be in the error
}
3

Is this for debugging during development or at runtime (e.g. to print to some log)?

For the former, you should be able to break on swift_willThrow in lldb.

For the latter, you use a wrapper error type and a helper function to wrap errors thrown from the functions you call:

struct SourceLocationError<T: Error>: Error {
    let file: String, line: Int, error: T
}

func annotateError<T>(_ proc: @autoclosure () throws -> T, file: String = #file, line: Int = #line) throws -> T {
    do {
        return try proc()
    } catch {
        throw SourceLocationError(file: file, line: line, error: error)
    }
}

And then use it like this:

do {
    try annotateError(someFunction())
} catch {
    print(error)
}

Sadly, from what I tried, pre-/postfix operators cant have extra arguments, even when they have default values. That would have made it a bit less visually obtrusive, but if that is an issue a shorter wrapper function name should go a long way.

1 Like
4

Hi
@2, A call stack would be nice, Although I don't mind just the file and line number
@3, Yes it's for loggingز
All the ways shown here seems cool just except that I have to write an extra function for everything that errors, So I wonder if there's something easier?
I just want whenever I have an error raised I can log it in a way so I know where it happened, I rarely handle errors within the inner functions and just make it go to the highest function where main is because it works for my use case.

5

Nice!

Extending on your example (with recording "a stack" of thrown locations plus function names):

import Foundation

struct FileLocation: CustomStringConvertible {
    let file: String
    let line: Int
    let function: String

    var description: String {
        "\(file):\(line) \(function)"
    }
}

struct SourceLocationError: Error, CustomStringConvertible {
    var fileLocations: [FileLocation]
    let error: Error
    
    var description: String {
        let locs = fileLocations.reduce("") { e, loc in
            e.appending("\(loc)\n")
        }
        return "\(error)\n\(locs)"
    }
}

func annotateError<T>(_ proc: @autoclosure () throws -> T, file: String = #fileID, line: Int = #line, function: String = #function) throws -> T {
    do {
        return try proc()
    } catch {
        if let error = error as? SourceLocationError {
            var error = error
            error.fileLocations.append(FileLocation(file: file, line: line, function: function))
            throw error
        } else {
            throw annotatedError(error, file: file, line: line, function: function)
        }
    }
}

func annotatedError(_ error: Error, file: String = #fileID, line: Int = #line, function: String = #function) -> Error {
    SourceLocationError(fileLocations: [FileLocation(file: file, line: line, function: function)], error: error)
}

enum SomeError: Error {
    case someError
    case someOtherError
}

func foo(with: String, and: Int) throws {
    throw annotatedError(SomeError.someError)
    // throw SomeError.someError
}

func bar() throws {
    try annotateError(foo(with: "", and: 1))
}

func baz(with: String) throws {
    try annotateError(bar())
}

func test() {
    do {
        try annotateError(baz(with: "Hello"))
    }
    catch {
        let err = error
        print(err)
    }
}

test()

Gives this output:

someError
JT/main.swift:51 foo(with:and:)
JT/main.swift:56 bar()
JT/main.swift:60 baz(with:)
JT/main.swift:65 test()

without using annotatedError when throwing the error "call stack" misses the entry for "foo" but is still reasonable:

someError
JT/main.swift:56 bar()
JT/main.swift:60 baz(with:)
JT/main.swift:65 test()

The file locations are spot on👌:

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1 Like