I have an array of String and need to call withCString on each one and then pass the resulting array of UnsafePointer<Int8> to a C api. The unsafe pointer is only valid within the scope of withCString.
So I built the following, but the compiler doesn’t like it:
% swift scopedReduceAndApply.swift
scopedReduceAndApply.swift:18:16: error: value of type 'Self.SubSequence' has no member 'Distance'
return tail.scopedReduceAndApply(f, g: g, list: ll)
^~~~ ~~~~~~~~~~~~~~~~~~~~
What is ‘Distance’ and why doesn’t SubSequence of CollectionType have it?
/Daniel
extension CollectionType where SubSequence : CollectionType {
/// Calls `f` for each Element to (scoped) generate a value of type `A`
/// and pass it to the given function (3rd argument to `f`).
/// The array of all generated `A` values will then be passed to the function
/// `g`. Its return value will be returned by `scopedReduce`.
///
/// This is similar to `reduce` but the function `g` is called while all calls
/// to `f` are on the stack, i.e. are in scope. This allows `f` to call a
/// function such as `String.withCString` and ensure that we're in its scope.
private func scopedApply<A,B>(f: (Generator.Element, (A) -> B) -> B, g: [A] -> B) -> B {
return scopedReduceAndApply(f, g: g, list: )
}
private func scopedReduceAndApply<A,B>(f: (Generator.Element, (A) -> B) -> B, g: [A] -> B, list: [A]) -> B {
if let (head, tail) = decompose {
return f(head) { (a: A) -> B in
var ll = list
ll.append(a)
return tail.scopedReduceAndApply(f, g: g, list: ll)
}
} else {
return g(list)
}
}
}
I have an array of String and need to call withCString on each one and then pass the resulting array of UnsafePointer<Int8> to a C api. The unsafe pointer is only valid within the scope of withCString.
So I built the following, but the compiler doesn’t like it:
% swift scopedReduceAndApply.swift
scopedReduceAndApply.swift:18:16: error: value of type 'Self.SubSequence' has no member 'Distance'
return tail.scopedReduceAndApply(f, g: g, list: ll)
^~~~ ~~~~~~~~~~~~~~~~~~~~
What is ‘Distance’ and why doesn’t SubSequence of CollectionType have it?
That's just a bad error message.
extension CollectionType where SubSequence : CollectionType {
I think you need this constraint to be propagated all the way down,
for recursive calls, so you need:
extension CollectionType where SubSequence == Self
Or, if you want to allow the toplevel type to differ:
extension CollectionType
where
SubSequence : CollectionType,
SubSequence.SubSequence == SubSequence
Dmitri
···
On Wed, Dec 23, 2015 at 3:24 PM, Daniel Eggert via swift-users <swift-users@swift.org> wrote:
On 24 Dec 2015, at 03:11, Dmitri Gribenko <gribozavr@gmail.com> wrote:
On Wed, Dec 23, 2015 at 3:24 PM, Daniel Eggert via swift-users > <swift-users@swift.org> wrote:
I have an array of String and need to call withCString on each one and then pass the resulting array of UnsafePointer<Int8> to a C api. The unsafe pointer is only valid within the scope of withCString.
So I built the following, but the compiler doesn’t like it:
% swift scopedReduceAndApply.swift
scopedReduceAndApply.swift:18:16: error: value of type 'Self.SubSequence' has no member 'Distance'
return tail.scopedReduceAndApply(f, g: g, list: ll)
^~~~ ~~~~~~~~~~~~~~~~~~~~
What is ‘Distance’ and why doesn’t SubSequence of CollectionType have it?
That's just a bad error message.
extension CollectionType where SubSequence : CollectionType {
I think you need this constraint to be propagated all the way down,
for recursive calls, so you need:
extension CollectionType where SubSequence == Self
Or, if you want to allow the toplevel type to differ:
extension CollectionType
where
SubSequence : CollectionType,
SubSequence.SubSequence == SubSequence