RFC: template-haskell & Data.Map vs. Prelude.lookup use

[Austin Seipp]

Really, the patch is fine just as it is probably.

Earlier me and Herbert were wondering if anyone would actually be
affected by the O(n) lookup, but the only reason *I* really cared to
know that is just so we don't have to inline part of `containers` if
we don't have to. But at the end of the day it's barely 100 lines of
extra code. So maybe not that big a deal (but kinda lame).

To be on the conservative side and not regress (and this could easily
be considered a regression from a performance POV) we can certainly
just commit this and see if we can make the code smaller later, I'd
say.

On Fri, Apr 25, 2014 at 8:33 AM, Richard Eisenberg <eir at cis.upenn.edu> wrote:
>
> On Apr 25, 2014, at 4:16 AM, Herbert Valerio Riedel <hvriedel at gmail.com> wrote:
>
>> Hello Richard,
>>
>> On 2014-04-24 at 15:04:55 +0200, Richard Eisenberg wrote:
>>> That map seems to store the set of variables during printing TH, for
>>> the purposes of disambiguating identifiers with the same name but
>>> different uniques. If blatting out a whole lot of program text, I
>>> could imagine the Map getting somewhat sizeable.
>>
>> When does printing TH actually occur? If it doesn't occur during regular
>> compilation, do we ever need to pretty print large amounts of TH?
>
> I guess it depends on who “we” are. In the process of routine TH hackery, I would say that printing is uncommon -- normally the TH is processed and then just spliced, without user interaction. But, I can conceive of a library that does some processing and then prints.
>
>>
>>> But, it seems to only need the lookup and insert operations...
>
> Good observation.
>
>>
>> actually, it uses a specific combination of lookup+insert, so that it
>> would suffice to have the single operation in the style of Python's
>> dict.setdefault(), i.e.
>>
>>  findWithDefaultInsert :: Ord k => k -> v -> Map k v -> (v, Map k v)
>>  findWithDefaultInsert k default m
>>    | Just v <- Map.lookup k m = (v, m)
>>    | otherwise = default `seq` (default, Map.insert k default m)
>>
>>> is there a simpler data structure that has only these operations
>>> efficiently?
>>
>
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-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/