[swift-users] withUnsafeMutableBytes is killing me

[Ole Begemann]

On 25.04.2017 12:24, Rick Mann via swift-users wrote:
> Not the ResultType, you mean, but the input type, right?

Yes, sorry, I meant ContentType.

> Yeah, I finally figured that out, although it doesn't explain another situation I'm experiencing that I didn't include in the post.
>
> However, that doesn't explain why it can't infer it in the last example.
>
>> On Apr 25, 2017, at 02:58 , Ole Begemann <ole at oleb.net> wrote:
>>
>> The withUnsafeMutableBytes method has two generic parameters, ResultType and ContentType:
>>
>> mutating func withUnsafeMutableBytes<ResultType, ContentType>(_ body: (UnsafeMutablePointer<ContentType>) throws -> ResultType) rethrows -> ResultType
>>
>> In your examples, the type checker can't infer the type of ResultType. You'll have to state it explicitly by specifying the type of the closure's argument. For example:
>>
>> msg.withUnsafeMutableBytes {
>>      (inPointer
>> : UnsafeMutablePointer<UInt8>
>> ) -> Void in
>>      // ...
>> }
>>
>>
>> On 25.04.2017 10:45, Rick Mann via swift-users wrote:
>>> The following playground reproduces an issue I'm having, in that the code won't compile depending on the content of the closure. In fact, an empty closure is fine, but when I try to call certain things, it's not.
>>>
>>> I figure it has something to do with the type inference for inPointer, but I can't figure out what it needs to work.
>>>
>>> ---------------------------
>>> import Foundation
>>>
>>> //	OKAY:
>>>
>>> var msg = Data(capacity: 123456)
>>> msg.withUnsafeMutableBytes
>>> { (inPointer) -> Void in
>>> 	foo(inPointer)
>>> }
>>>
>>> //error: cannot convert value of type '(_) -> Void' to expected argument type '(UnsafeMutablePointer<_>) -> _'
>>> //{ (inPointer) -> Void in
>>> //^~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> msg.withUnsafeMutableBytes
>>> { (inPointer) -> Void in
>>> }
>>>
>>> //error: cannot convert value of type '(_) -> Void' to expected argument type '(UnsafeMutablePointer<_>) -> _'
>>> //{ (inPointer) -> Void in
>>> //^~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> msg.withUnsafeMutableBytes
>>> { (inPointer) -> Void in
>>> 	var s: Int
>>> 	lgs_error(inPointer, 123456, &s)
>>> }
>>>
>>> func
>>> foo(_ data: UnsafeMutableRawPointer!)
>>> {
>>> }
>>>
>>> func
>>> lgs_error(_ message: UnsafeMutablePointer<Int8>!,
>>> 	    _ message_capacity: Int,
>>> 	    _ message_size: UnsafeMutablePointer<Int>!) -> Int
>>> {
>>> }
>>> ---------------------------