[Pre-Draft] Nil-coalescing and errors

Evolution Discussion
1

Why not have throwing variants of the string and int methods (in addition to those returning an optional)?

Then you could simply write:

// Decodes a JSON with SwiftyJSON
do {
    let person: Person = try Person(
       firstName: json["firstName"].string,
      lastName: json["lastName"].string,
       age: json["age"].int)
} catch _ {
    // Error handling
}

-Thorsten

···

Am 07. April 2016 um 14:01 schrieb Yuta Koshizawa via swift-evolution swift-evolution@swift.org:

I'd like to see some real-world examples of this before we did anything with it.
I think I'm with Sean on this one. Optionals and throwing don't have enough
to do with each other to actually come up with a specific operator or method
for this. I can't help but see this as two ideas glued together:

  • "By this point in my execution I need a non-optional value, otherwise
    ______"
  • "_____ happened, therefore execution has failed and I should throw an
    error"

…and I'm not sure these ideas coincide enough to be worth gluing together.
There are a lot of other ways to get a non-optional value out of an optional
('??', '!', and 'guard let' with some other action), and there are a lot of
other ways to fail besides an optional being nil (status code came back as
error, unexpected data, connection timeout).

I'd like to see some real-world examples of this before we did anything with
it.

Jordan

On Apr 6, 2016, at 8:00, Sean Heber via swift-evolution > > swift-evolution@swift.org wrote:

Interesting, but I’m unsure if all of it is significantly better than just
using the guard that is effectively inside of the operator/func that is
being proposed:

guard let value = Int("NotANumber") else { throw
InitializerError.invalidString }

It is only a couple of characters longer and already works (it’s what I use
currently). If guard allowed for a special single-expression variation so
that you didn’t need to specify the ugly braces or something, it’d look
prettier and be nice for a lot of other situations, too:

guard let value = Int("NotANumber") else: throw
InitializerError.invalidString
guard someVal < 10 else: return false
guard mustBeTrue() else: return
// etc

Not to derail this, but I sort of want this ability anywhere as a shorthand
for a single-expression block.

if something < 42: doThing()
for a in list: print(a)

But I imagine that’ll never fly. :P

l8r
Sean

On Apr 6, 2016, at 9:46 AM, Erica Sadun via swift-evolution > > swift-evolution@swift.org wrote:

Pyry Jahkola and I have been plugging away on the following which is
preliminary enough not to qualify as an actual draft. He prefers the Mike
Ash approach. I prefer the operator approach. So we have not actually
settled on which one we would actually propose despite how I've written this
up.

I'm putting this out there to try to gain a consensus on:

  • Would this be a viable proposal?
  • If so, which of the options would work best within Swift's design and
    philosophy

Thanks for your feedback.

-- Erica
Introduction

Swift's try? keyword transforms error-throwing operations into optional
values. We propose adding an error-throwing nil-coalescing operator to the
Swift standard library. This operator will coerce optional results into
Swift's error-handling system.

This proposal was discussed on the Swift Evolution list in the name thread.

Motivation

Any decision to expand Swift's set of standard operators should be taken
thoughtfully and judiciously. Moving unaudited or deliberately
non-error-handling nil-returning methods and failable initializers into
Swift's error system should be a common enough use case to justify
introducing a new operator.

Detail Design

We propose adding a new operator that works along the following lines:

infix operator ??? {}

func ???(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {
guard case let value? = lhs else { throw error() }
return value
}

The use-case would look like this:

do {
let error = Error(reason: "Invalid string passed to Integer initializer")
let value = try Int("NotANumber") ??? InitializerError.invalidString
print("Value", value)
} catch { print(error) }

Note

SE-0047 (warn unused result by default) and SE-0049 (move autoclosure) both
affect many of the snippets in this proposal

Disadvantages to this approach:

• It consumes a new operator, which developers must be trained to use
• Unlike many other operators and specifically ??, this cannot be chained.
There's no equivalent to a ?? b ?? c ?? dor a ?? (b ?? (c ?? d)).
Alternatives Considered

Extending Optional

The MikeAsh approach extends Optional to add an orThrow(ErrorType) method

extension Optional {
func orThrow(@autoclosure error: () -> ErrorType) throws -> Wrapped {
guard case let value? = self else { throw error() }
return value
}
}

Usage looks like this:

do {
let value = try Int("NotANumber")
.orThrow(InitializerError.invalidString)
print("Value", value)
} catch { print(error) }

An alternative version of this call looks like this: optionalValue.or(throw:
error). I am not a fan of using a verb as a first statement label.

Disadvantages:

• Wordier than the operator, verging on claustrophobic, even using Swift's
newline dot continuation.
• Reading the code can be confusing. This requires chaining rather than
separating error throwing into a clear separate component.
Advantages:

• No new operator, which maintains Swift operator parsimony and avoids the
introduction and training issues associated with new operators.
• Implicit Optional promotion cannot take place. You avoid mistaken usage
like nonOptional ??? error and nonOptional ?? raise(error).
• As a StdLib method, autocompletion support is baked in.
Introducing a StdLib implementation of raise(ErrorType)

Swift could introduce a raise(ErrorType) -> T global function:

func raise(error: ErrorType) throws -> T { throw error }

do {
let value = try Int("NotANumber") ??
raise(InitializerError.invalidString)
print("Value", value)
} catch { print(error) }

This is less than ideal:

• This approach is similar to using && as an if-true condition where an
operator is abused for its side-effects.
• It is wordier than the operator approach.
• The error raising function promises to return a type but never will, which
seems hackish.
Overriding ??

We also considered overriding ?? to accept an error as a RHS argument. This
introduces a new way to interpret ?? as meaning, "throw this error instead
of substituting this value".

func ??(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {
guard case let value? = lhs else { throw error() }
return value
}

Usage:

let value = try Int("NotANumber") ?? Error(reason: "Invalid string passed to
Integer initializer")

This approach overloads the semantics as well as the syntax of the
coalescing operator. Instead of falling back to a RHS value, it raises the
RHS error. The code remains simple and readable although the developer must
take care to clarify through comments and naming which version of the
operator is being used.

• While using try in the ?? statement signals that a throwing call is in
use, it is insufficient (especially when used in a throwing scope) to
distinguish between the normal coalescing and new error-throwing behaviors.
• Error types need not use the word "Error" in their construction or use.
For example try value ?? e may not be immediately clear as an error-throwing
intent.
• Overloading ?? dilutes the impact and meaning of the original operator
intent.
Future Directions

We briefly considered something along the lines of perl's die as an
alternative to raise using fatalError.

Acknowledgements

Thanks Mike Ash, Jido, Dave Delong

swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

The following is my real-world example.

// Decodes a JSON with SwiftyJSON
do {
 let person: Person = try Person(
 firstName: json["firstName"].string ??? Error(),
 lastName: json["lastName"].string ??? Error(),
 age: json["age"].int ??? Error()
 )
} catch _ {
 // Error handling
}

With guard, we have to repeat the parameter names meaninglessly.

do {
 guard let firstName = json["firstName"].string else { throw Error() }
 guard let lastName = json["lastName"].string else { throw Error() }
 guard let age = json["age"].string else { throw Error() }
 let person: Person = Person(firstName: firstName, lastName:
lastName, age: age)
} catch _ {
 // Error handling
}

guard is a statement. ??? makes an expression. Expressions are
useful when we want to pass their return values as arguments directly.
I think ??? is valuable to get an unwrapped value or throw an error
as an expression.

-- Yuta

2016-04-07 2:45 GMT+09:00 Jordan Rose via swift-evolution
swift-evolution@swift.org:

swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

2

Of course we can implement a library with `throws` by changing those
properties to methods. However we can think about similar cases.

do {
    let foo: Foo = try foo(
        b: Int(bString) ??? Error(),
        c: Int(cString) ??? Error()
    )
} catch _ {
    // Error handling
}

`Dictionary` also returns optionals. We have already had various
functions/methods which returns optionals.

-- Yuta

···

a: Int(aString) ??? Error(),

2016-04-07 21:34 GMT+09:00 Thorsten Seitz via swift-evolution <swift-evolution@swift.org>:

Why not have throwing variants of the `string` and `int` methods (in
addition to those returning an optional)?

Then you could simply write:

// Decodes a JSON with SwiftyJSON
do {
   let person: Person = try Person(
      firstName: json["firstName"].string,
      lastName: json["lastName"].string,
      age: json["age"].int)
} catch _ {
    // Error handling
}

-Thorsten

Am 07. April 2016 um 14:01 schrieb Yuta Koshizawa via swift-evolution > <swift-evolution@swift.org>:

I'd like to see some real-world examples of this before we did anything with
it.

The following is my real-world example.

// Decodes a JSON with SwiftyJSON
do {
let person: Person = try Person(
firstName: json["firstName"].string ??? Error(),
lastName: json["lastName"].string ??? Error(),
age: json["age"].int ??? Error()
)
} catch _ {
// Error handling
}

With `guard`, we have to repeat the parameter names meaninglessly.

do {
guard let firstName = json["firstName"].string else { throw Error() }
guard let lastName = json["lastName"].string else { throw Error() }
guard let age = json["age"].string else { throw Error() }
let person: Person = Person(firstName: firstName, lastName:
lastName, age: age)
} catch _ {
// Error handling
}

`guard` is a statement. `???` makes an expression. Expressions are
useful when we want to pass their return values as arguments directly.
I think `???` is valuable to get an unwrapped value or throw an error
as an expression.

-- Yuta

2016-04-07 2:45 GMT+09:00 Jordan Rose via swift-evolution
<swift-evolution@swift.org>:

I think I'm with Sean on this one. Optionals and throwing don't have enough

to do with each other to actually come up with a specific operator or method

for this. I can't help but see this as two ideas glued together:

- "By this point in my execution I need a non-optional value, otherwise

______"

- "_____ happened, therefore execution has failed and I should throw an

error"

…and I'm not sure these ideas coincide enough to be worth gluing together.

There are a lot of other ways to get a non-optional value out of an optional

('??', '!', and 'guard let' with some other action), and there are a lot of

other ways to fail besides an optional being nil (status code came back as

error, unexpected data, connection timeout).

I'd like to see some real-world examples of this before we did anything with

it.

Jordan

On Apr 6, 2016, at 8:00, Sean Heber via swift-evolution > > <swift-evolution@swift.org> wrote:

Interesting, but I’m unsure if all of it is significantly better than just

using the guard that is effectively inside of the operator/func that is

being proposed:

guard let value = Int("NotANumber") else { throw

InitializerError.invalidString }

It is only a couple of characters longer and already works (it’s what I use

currently). If guard allowed for a special single-expression variation so

that you didn’t need to specify the ugly braces or something, it’d look

prettier and be nice for a lot of other situations, too:

guard let value = Int("NotANumber") else: throw

InitializerError.invalidString

guard someVal < 10 else: return false

guard mustBeTrue() else: return

// etc

Not to derail this, but I sort of want this ability anywhere as a shorthand

for a single-expression block.

if something < 42: doThing()

for a in list: print(a)

But I imagine that’ll never fly. :P

l8r

Sean

On Apr 6, 2016, at 9:46 AM, Erica Sadun via swift-evolution > > <swift-evolution@swift.org> wrote:

Pyry Jahkola and I have been plugging away on the following which is

preliminary enough not to qualify as an actual draft. He prefers the Mike

Ash approach. I prefer the operator approach. So we have not actually

settled on which one we would actually propose despite how I've written this

up.

I'm putting this out there to try to gain a consensus on:

* Would this be a viable proposal?

* If so, which of the options would work best within Swift's design and

philosophy

Thanks for your feedback.

-- Erica

Introduction

Swift's try? keyword transforms error-throwing operations into optional

values. We propose adding an error-throwing nil-coalescing operator to the

Swift standard library. This operator will coerce optional results into

Swift's error-handling system.

This proposal was discussed on the Swift Evolution list in the name thread.

Motivation

Any decision to expand Swift's set of standard operators should be taken

thoughtfully and judiciously. Moving unaudited or deliberately

non-error-handling nil-returning methods and failable initializers into

Swift's error system should be a common enough use case to justify

introducing a new operator.

Detail Design

We propose adding a new operator that works along the following lines:

infix operator ??? {}

func ???<T>(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {

guard case let value? = lhs else { throw error() }

return value

}

The use-case would look like this:

do {

let error = Error(reason: "Invalid string passed to Integer initializer")

let value = try Int("NotANumber") ??? InitializerError.invalidString

print("Value", value)

} catch { print(error) }

Note

SE-0047 (warn unused result by default) and SE-0049 (move autoclosure) both

affect many of the snippets in this proposal

Disadvantages to this approach:

• It consumes a new operator, which developers must be trained to use

• Unlike many other operators and specifically ??, this cannot be chained.

There's no equivalent to a ?? b ?? c ?? dor a ?? (b ?? (c ?? d)).

Alternatives Considered

Extending Optional

The MikeAsh approach extends Optional to add an orThrow(ErrorType) method

extension Optional {

func orThrow(@autoclosure error: () -> ErrorType) throws -> Wrapped {

guard case let value? = self else { throw error() }

return value

}

}

Usage looks like this:

do {

let value = try Int("NotANumber")

.orThrow(InitializerError.invalidString)

print("Value", value)

} catch { print(error) }

An alternative version of this call looks like this: optionalValue.or(throw:

error). I am not a fan of using a verb as a first statement label.

Disadvantages:

• Wordier than the operator, verging on claustrophobic, even using Swift's

newline dot continuation.

• Reading the code can be confusing. This requires chaining rather than

separating error throwing into a clear separate component.

Advantages:

• No new operator, which maintains Swift operator parsimony and avoids the

introduction and training issues associated with new operators.

• Implicit Optional promotion cannot take place. You avoid mistaken usage

like nonOptional ??? error and nonOptional ?? raise(error).

• As a StdLib method, autocompletion support is baked in.

Introducing a StdLib implementation of raise(ErrorType)

Swift could introduce a raise(ErrorType) -> T global function:

func raise<T>(error: ErrorType) throws -> T { throw error }

do {

let value = try Int("NotANumber") ??

raise(InitializerError.invalidString)

print("Value", value)

} catch { print(error) }

This is less than ideal:

• This approach is similar to using && as an if-true condition where an

operator is abused for its side-effects.

• It is wordier than the operator approach.

• The error raising function promises to return a type but never will, which

seems hackish.

Overriding ??

We also considered overriding ?? to accept an error as a RHS argument. This

introduces a new way to interpret ?? as meaning, "throw this error instead

of substituting this value".

func ??<T>(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {

guard case let value? = lhs else { throw error() }

return value

}

Usage:

let value = try Int("NotANumber") ?? Error(reason: "Invalid string passed to

Integer initializer")

This approach overloads the semantics as well as the syntax of the

coalescing operator. Instead of falling back to a RHS value, it raises the

RHS error. The code remains simple and readable although the developer must

take care to clarify through comments and naming which version of the

operator is being used.

• While using try in the ?? statement signals that a throwing call is in

use, it is insufficient (especially when used in a throwing scope) to

distinguish between the normal coalescing and new error-throwing behaviors.

• Error types need not use the word "Error" in their construction or use.

For example try value ?? e may not be immediately clear as an error-throwing

intent.

• Overloading ?? dilutes the impact and meaning of the original operator

intent.

Future Directions

We briefly considered something along the lines of perl's die as an

alternative to raise using fatalError.

Acknowledgements

Thanks Mike Ash, Jido, Dave Delong

_______________________________________________

swift-evolution mailing list

swift-evolution@swift.org

https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________

swift-evolution mailing list

swift-evolution@swift.org

https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________

swift-evolution mailing list

swift-evolution@swift.org

https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution