[swift-users] withUnsafeMutableBytes is killing me

[Rick Mann]

Not the ResultType, you mean, but the input type, right? 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
>> {
>> }
>> ---------------------------
>> 


-- 
Rick Mann
rmann at latencyzero.com