POLA fetch

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to fetch(key, absentThunk)

The fetch/2 protocol for collections does not obey POLA:

  • The collection gets access to the value of absentThunk.
  • The collection may invoke absentThunk and yet return something else.
  • The collection may invoke absentThunk more than once, or after fetch returns.


  • Make the second argument usually an ejector. Ejectors do not return values and are robust against being invoked multiple times; if some other sort of object is being used, its author is more likely to consider the multiple invocation issue. --Kevin Reid 11:25, 12 April 2008 (CDT)
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