POLA fetch
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Kevin Reid (Talk | contribs) (publishing scribbles sitting on my disk) |
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# works with existing fetch implementations | # works with existing fetch implementations | ||
def value := [[escape]] e { | def value := [[escape]] e { | ||
- | map.[[Message fetch/2|fetch]](key, e) | + | map.[[:Category:Message fetch/2|fetch]](key, e) |
} catch _ { | } catch _ { | ||
map[key] := makeValue() | map[key] := makeValue() |
Latest revision as of 17:15, 11 August 2008
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.
Proposals
- 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)
Scribbles
# 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() } }