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. If we do this, then utilities should be provided for default-value behavior. --Kevin Reid 11:25, 12 April 2008 (CDT)


# Current example (ensuring FlexMap entry, has the POLA problem discussed here)
def value := map.fetch(key, fn {
               map[key] := makeValue()

# works with existing fetch implementations
def value := escape e { 
               map.fetch(key, e)
             } catch _ { 
               map[key] := makeValue()

# return-value-distinguishing #1
def value := switch (map.fetch(key)) {
               match [value] { value }
               match [] { map[key] := makeValue() }

# return-value-distinguishing #2, using call-patterns and Haskell-style Maybe
def value := switch (map.fetch(key)) {
               match just(value) { value }
               match nothing()   { map[key] := makeValue() }
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