POLA fetch

From Erights

(Difference between revisions)
Jump to: navigation, search
m (whoops, inconsistent links)
Line 9: Line 9:
== Proposals ==
== 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. --[[User:Kevin Reid|Kevin Reid]] 11:25, 12 April 2008 (CDT)
+
* 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. --[[User:Kevin Reid|Kevin Reid]] 11:25, 12 April 2008 (CDT)
-
 
+
-
== Scribbles ==
+
-
 
+
-
# Current example (ensuring [[FlexMap]] entry, has the POLA problem discussed here)
+
-
def value := map.[[:Category:Message fetch/2|fetch]](key, fn {
+
-
                map[key] := makeValue()
+
-
              })
+
-
+
-
# works with existing fetch implementations
+
-
def value := [[escape]] e {
+
-
                map.[[:Category:Message fetch/2|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() }
+
-
              }
+
[[Category:Unresolved design issues]]
[[Category:Unresolved design issues]]
-
[[Category:Message fetch/2]]
 
-
[[Category:Message fetch/1]]
 

Revision as of 16:25, 12 April 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. --Kevin Reid 11:25, 12 April 2008 (CDT)
Retrieved from "http://wiki.erights.org/wiki/POLA_fetch"
Personal tools
more tools