Re: [swift-users] withUnsafeMutableBytes is killing me

[Ole Begemann via swift-users]

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 <o...@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
{
}
---------------------------

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