functional programming - Ambiguous use of functions (Swift 4) -


i've been trying figure out why following code ambiguous...

public func product <t1: sequence, t2: sequence> (_ sequence1: t1, _ sequence2: t2) ->     anysequence<(t1.element, t2.element)> {   return anysequence(     sequence1.flatmap { element1 in       sequence2.map { element2 in         (element1, element2)       }     }   ) }  public func product <t1: lazysequenceprotocol, t2: lazysequenceprotocol> (_ sequence1: t1, _ sequence2: t2) ->     lazysequence<anysequence<(t1.element, t2.element)>> {   return anysequence(     sequence1.flatmap { element1 in       sequence2.map { element2 in         (element1, element2)       }     }   ).lazy }

...when call 2 lazy sequences , call makeiteratorexample.

_ = product([1, 2].lazy, [3, 4].lazy).makeiterator()

yet, following code doesn't have ambiguity...

public struct product2sequence <t1: sequence, t2: sequence>: sequence {   public typealias element = (t1.element, t2.element)   public typealias iterator = anyiterator<element>    private let iterator: iterator    internal init (_ sequence1: t1, _ sequence2: t2) {     self.iterator = anyiterator(       sequence1.flatmap { element1 in         sequence2.map { element2 in           (element1, element2)         }       }.makeiterator()     )   }    public func makeiterator () -> iterator {     return self.iterator   } }  public struct lazyproduct2sequence <t1: lazysequenceprotocol, t2: lazysequenceprotocol>: lazysequenceprotocol {   public typealias element = (t1.element, t2.element)   public typealias iterator = anyiterator<element>    private let iterator: iterator    internal init (_ sequence1: t1, _ sequence2: t2) {     self.iterator = anyiterator(       sequence1.flatmap { element1 in         sequence2.map { element2 in           (element1, element2)         }       }.makeiterator()     )   }    public func makeiterator () -> iterator {     return self.iterator   } }  public func product <t1: sequence, t2: sequence> (_ sequence1: t1, _ sequence2: t2) -> product2sequence<t1, t2> {   return product2sequence(sequence1, sequence2) }  public func product <t1: lazysequenceprotocol, t2: lazysequenceprotocol> (_ sequence1: t1, _ sequence2: t2) ->     lazyproduct2sequence<t1, t2> {   return lazyproduct2sequence(sequence1, sequence2) }

...when call 2 lazy sequences , call makeiteratorexample.

_ = product([1, 2].lazy, [3, 4].lazy).makeiterator()

my reasoning lazy sequence conforms both lazysequenceprotocol , sequence type system doesn't know product choose. definition second version should not work.

i'm using swift 4.0.

what makes second version work?


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