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digitalmars.D - Associative array mutation of a value

reply "bearophile" <bearophileHUGS lycos.com> writes:
Recently Erlang language has added several things:
http://joearms.github.io/2014/02/01/big-changes-to-erlang.html

One interesting change is the syntax to handle associative arrays 
(that are meant to be used a little like records):


 Updating a map is done as follows:
 



X is the old associative array. "=>" adds a mew key, while ":=" 
updates an already existing key, and if it's not present an error 
is generated. So a spelling mistake cannot accidentally introduce 
a new key if you want to update some value. This avoids a common 
mistake.

I am not suggesting to add a new syntax to D, but perhaps a 
simple "AA.update(key, val)" function in the object module can be 
useful.

Bye,
bearophile
Feb 06 2014
next sibling parent reply "Tobias Pankrath" <tobias pankrath.net> writes:
On Thursday, 6 February 2014 at 14:15:47 UTC, bearophile wrote:

 Recently Erlang language has added several things:
 http://joearms.github.io/2014/02/01/big-changes-to-erlang.html

 One interesting change is the syntax to handle associative 
 arrays (that are meant to be used a little like records):


 Updating a map is done as follows:
 



 X is the old associative array. "=>" adds a mew key, while ":=" 
 updates an already existing key, and if it's not present an 
 error is generated. So a spelling mistake cannot accidentally 
 introduce a new key if you want to update some value. This 
 avoids a common mistake.

 I am not suggesting to add a new syntax to D, but perhaps a 
 simple "AA.update(key, val)" function in the object module can 
 be useful.

 Bye,
 bearophile



AA[key] = val; ?
Feb 06 2014
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Tobias Pankrath:


 AA[key] = val; ?


I don't understand. That syntax doesn't give you an error if 
'key' is missing in AA.

Bye,
bearophile
Feb 06 2014
parent "Tobias Pankrath" <tobias pankrath.net> writes:
On Thursday, 6 February 2014 at 15:16:14 UTC, bearophile wrote:

 Tobias Pankrath:


 AA[key] = val; ?


 I don't understand. That syntax doesn't give you an error if 
 'key' is missing in AA.

 Bye,
 bearophile


yes, brainfart. For a moment I thought that the index would throw 
if key is missing, return a reference otherwise and assign val to 
it. That's obviously wrong.
Feb 06 2014
prev sibling parent reply "Idan Arye" <GenericNPC gmail.com> writes:
On Thursday, 6 February 2014 at 14:15:47 UTC, bearophile wrote:

 Recently Erlang language has added several things:
 http://joearms.github.io/2014/02/01/big-changes-to-erlang.html

 One interesting change is the syntax to handle associative 
 arrays (that are meant to be used a little like records):


 Updating a map is done as follows:
 



 X is the old associative array. "=>" adds a mew key, while ":=" 
 updates an already existing key, and if it's not present an 
 error is generated. So a spelling mistake cannot accidentally 
 introduce a new key if you want to update some value. This 
 avoids a common mistake.

 I am not suggesting to add a new syntax to D, but perhaps a 
 simple "AA.update(key, val)" function in the object module can 
 be useful.

 Bye,
 bearophile


I'm always for little helper functions, but I don't think 
Erlang's idiom of using maps as records is good for D. That model 
works well with dynamic typing, where record types are 
meaningless and only their structure is important(JavaScript is 
fine example). With static typing it's better to declare a type 
for the record, so whether the field you are trying to set exists 
or not can be checked at compile-time.

Besides, the usefulness of maps-as-records will be quite limited 
in D, since associative arrays has a fixed value type. To store 
different types in different fields, we'll need to stringly-type 
them(very bad!), use pointers abd casting(not that bad but still 
bad!) or use Variant - which is better than the first two 
solutions but still not as good as simply using structs or 
classes.
Feb 07 2014
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Idan Arye:


 but I don't think Erlang's idiom of using maps as records is 
 good for D.


I didn't meant to use that idiom in D, and I am not suggesting 
its usage in D. I wanted just to suggest an alternative way to 
modify associative array values that requires the keys to be 
already present. Because this is a common operation where you 
don't want to add a new key by mistake.

Bye,
bearophile
Feb 07 2014
prev sibling parent "renoX" <renozyx gmail.com> writes:
On Friday, 7 February 2014 at 20:22:54 UTC, Idan Arye wrote:

 I'm always for little helper functions, but I don't think 
 Erlang's idiom of using maps as records is good for D. That 
 model works well with dynamic typing, where record types are 
 meaningless and only their structure is important(JavaScript is 
 fine example). With static typing it's better to declare a type 
 for the record, so whether the field you are trying to set 
 exists or not can be checked at compile-time.


The thing is, you don't always have those key at compile time..

Anyway, the discussion here is about map not record and IMHO it 
is a good idea to have two short functions for 'replace an 
existing value' and 'add a new key' then a long one for 'do both' 
because users tend to use short names and from a maintainability 
point of view, it's much better to know what was the exact 
intended meaning of the one who wrote the code, instead of having 
to guess, he wrote "array[key] = value" did he want to add the 
key or did to replace an existing key?

BR,
renoX
Feb 10 2014