Swift lazy collection evaluated multiple times

Number:rdar://23697280 Date Originated:30-Nov-2015 04:33 PM
Status:Open Resolved:
Product:Developer Tools Product Version:Swift 2.1
Classification:Other Bug Reproducible:Always

Sometimes when accessing elements from a lazy collection, the element is
evaluated multiples times, resulting in unexpected function calls.

Steps to reproduce:

1. Run the attached example with
  `swift LazyCollectionEvaluatedMultipleTimes.swift`

Expected results:

The output is:

Called with 'a' returning ''
Called with 'b' returning 'hi'

Where each of these entries is evaluated only a single time.

Actual results:

The output is:

Called with 'a' returning ''
Called with 'b' returning 'hi'
Called with 'b' returning 'hi'

Where `Called with 'b' returning 'hi'` is because the element is
evaluated twice.


- This seems to be a side effect of accessing `self[index]` in the given
- This behavior might be intended, although the expensive function we
  were attempting to call lazily being called multiples times seemed



By keithbsmiley at Dec. 1, 2015, 12:34 a.m. (reply...)

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