[m-users.] Question about map internal representation for repeated values on different keys

[Sean Charles (emacstheviking)]

Beautiful!

I don’t need this feature at the moment, it would fall into the category of ‘premature optimisation’ given that the expected number of entries per translation phase per file is probably less than 50 (wild guess) declarations.

I will read this code and study it more closely this evening Zoltan.

Thanks again,
Sean.

> On 29 Nov 2021, at 23:04, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:
> 
> 
> 2021-11-30 09:36 GMT+11:00 "Sean Charles (emacstheviking)" <objitsu at gmail.com>:
>> What does map(string, string) do if I insert many keys that have the same value, does it re-share that string value?
> 
> No, it does not. What you are talking about is hash-consing, https://en.wikipedia.org/wiki/Hash_consing,
> and Mercury has no built-in mechanism for doing it.
> 
> However, that does not mean it cannot be done, and in fact it is easy to do: you just
> need to implement the hash-consing yourself. This is trivially done using a canonicalization map,
> which in this case would be a map from strings to strings, which would map any string to
> a "canonical instance" of that string. The code for that would be as simple as
> 
> :- pred canonicalize(T::in, T::out, map(T, T)::in, map(T, T)::out) is det.
> 
> canonicalize(X, CanonX, !Map) :-
>  ( if map.search(!.Map, X, CanonXPrime) then
>    CanonX = CanonXPrime
>  else
>    CanonX = X,
>     map.det_insert(X, CanonX, !Map)
>  ).
> 
> While this code is simple, it does have a performance cost in both time and space, which is why
> this is not done by default.
> 
> Zoltan.