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Dylan Design Notes: #14: Union Allows Duplicates (Clarification)

Dylan Design Notes

#14: Union Allows Duplicates (Clarification)

Version 1, March 1993

This design note clarifies that the result of union is only guaranteed free of duplicates if no element appears more than once in a single argument sequence. If the same element appears more than once in a single argument sequence, it may appear more than once in the result sequence. This allows union to be implemented more efficiently.


Replace the first sentence of the description of union on page 107 of the manual with the following:

union returns a sequence containing every element of sequence1 and sequence2. If the same element appears in both argument sequences, this will not cause it to appear twice in the result sequence. However, if the same element appears more than once in a single argument sequence, it may appear more than once in the result sequence.

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