index(_:offsetBy:limitedBy)...what's the type?

Question

Created Jan ’18

Replies 4

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Participants 3

I'm trying to get a simple index offset:

func foo<Bytes: Collection>(data: Bytes) where Bytes.Element == UInt8 {
    let nextFrameStartIndex = data.index(data.startIndex, offsetBy: 10, limitedBy: data.endIndex)
}

If I put the integer literal 10 there it compiles just fine. If I instead create a variable of type Int and pass that I get a compiler error:

"Cannot invoke 'index' with an argument list of type '(Bytes.Index, offsetBy: Int, limitedBy: Bytes.index)'

Answered by OOPer in 289384022

When an associated type is declared with `=`, the right hand side represents a default type for the associated type.

I'm not sure when exactly the `default` would be applied, so I cannot explain why your literal case works as expected.

But, generally, when you implement something with associated types, you should better explicitly specify them:

func foo<Bytes: Collection>(data: Bytes)
    where Bytes.Element == UInt8, Bytes.IndexDistance == Int
{
    let nextFrameStartIndex = data.index(data.startIndex, offsetBy: index, limitedBy: data.endIndex)
}

(Assume `index` is a variable of `Int`.)

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Answer 1

Claude31 OP

Jan ’18

I cannot find the thread where a similar bquestiçon was addressed.

I think I remember type to declare is not Int but another type of Integer (which, I can't remember).

Maybe FixedWidthInteger ?

0

Answer 2

Gargoyle OP

Jan ’18

Well, the header file declares it as Self.IndexDistance, and when I scroll up to the top of the collection protocol it has

associatedtype IndexDistance = Int

So you can see why I'm so confused 🙂

0

Answer 3

OOPer OP

Jan ’18

Accepted Answer

When an associated type is declared with `=`, the right hand side represents a default type for the associated type.

I'm not sure when exactly the `default` would be applied, so I cannot explain why your literal case works as expected.

But, generally, when you implement something with associated types, you should better explicitly specify them:

func foo<Bytes: Collection>(data: Bytes)
    where Bytes.Element == UInt8, Bytes.IndexDistance == Int
{
    let nextFrameStartIndex = data.index(data.startIndex, offsetBy: index, limitedBy: data.endIndex)
}

(Assume `index` is a variable of `Int`.)

0

Answer 4

Gargoyle OP

Jan ’18

Thank you! That never even occurred to me to try.

0