Containers and strictness

Claus Reinke claus.reinke at
Thu Jun 24 08:46:14 EDT 2010

>> >  As Claus wrote, right now it is undocumented and inconsistent (both in
>> >  the methods of one container and also in the same methods of different
>> >  container).
>> Just as it is sometimes important to be able to do strict inserts, it
>> is important sometimes that we have maps that are lazy in the
>> elements. There are important use cases both ways.
> I thought we are talking only about keys/elements. I would leave the
> values untouched.

There are problems at several levels:

1 it needs to be documented/specified to what extent one can
    rely on strictness; usually, Maps are
    - strict in keys and structure (so forcing the Map forces all keys),
    - not strict in values

2 it needs to be documented to what extent operations preserve
    or enforce strictness;

3 when the default strictness does not match the needs of the
    application, the library needs to provide sufficient hooks for
    enforcing different strictness policies (there might be a
    relation to the recent strategies work, btw);

All three points are currently undocumented, though 1 is assumed.
2 and 3 are terribly inconsistent at the moment: not only are there
missing strict-in-values versions of most operations, and some
versions only in Map, not in IntMap; the real problems start when
the existing operations simply do not provide the means to
enforce different strictness policies.

Things one wants to be able to do include:

- pass strict operations to higher-order API functions and have
    the strictness information propagated through the strictness

- tie value evaluation to key availability (since the Map is strict
    in the latter, it would then become strict in the former, too)

- preserve/push strictness policies through all API operations
    (don't have foldl's accumulating Maps blow up because of
    non-strictifiable Map combination operators; don't have
    inserts sometimes evaluate values, sometime not; don't
    have Map traversal operations in the API that give no handle
    for adding strictness)

As Duncan points out, both strict and non-strict versions
have their uses. Non-strict in values is a useful default, IMHO,
but one needs to be able to introduce strictness where required
(some have argued for strict in values as default, with a data
type indirection to re-introduce non-strictness).

The blowups I've seen usually involve values not being
evaluated through Map/IntMap operations (insert, fold,
union, ..). But as with foldr/foldl/fold', that does not mean
that everything should be strict, just that users need to be
able to specify as much or as little strictness as is appropriate
for their application.



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