[Ur] Default settings using a generic record update

[Mark Clements]

Ziv: thank you for this suggestion. Note that I have raised an issue on UrLib on an unrelated matter.

After some head-scratching, I got as far as:

    fun update_ab [write] (sub : Subset.t [A=int,B=int] write) (w : $write) =
        {A=1,B=2} --- write ++ w

which also does not work. Any guidance here would be appreciated - I admit to being stuck after looking closely at UrLib's subset.ur(s) and magicTable.ur(s).

Kindly, Mark.

On 29/10/19 4:39 pm, Ziv Scully wrote:
One can capture subset constraints using a type class, as the following module does:

https://github.com/vizziv/UrLib/blob/master/UrLib/subset.urs<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fvizziv%2FUrLib%2Fblob%2Fmaster%2FUrLib%2Fsubset.urs&data=02%7C01%7Cmark.clements%40ki.se%7C46a639348ebc4f9a241108d75c865133%7Cbff7eef1cf4b4f32be3da1dda043c05d%7C0%7C0%7C637079604764017394&sdata=ueZ7e9P7Rzqhq7P4vuBDpbcq8QbIYA5i8vhL9%2BgCvEc%3D&reserved=0>
https://github.com/vizziv/UrLib/blob/master/UrLib/subset.ur<https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fvizziv%2FUrLib%2Fblob%2Fmaster%2FUrLib%2Fsubset.ur&data=02%7C01%7Cmark.clements%40ki.se%7C46a639348ebc4f9a241108d75c865133%7Cbff7eef1cf4b4f32be3da1dda043c05d%7C0%7C0%7C637079604764027387&sdata=RDT9vPh4qegdK9iiBr%2F2K7vpFc4z%2BF2y4cHa2eDe5Tw%3D&reserved=0>

(It relies on another module, Eq, from the same project.)

Ziv


On Tue, Oct 29, 2019 at 11:21 Mark Clements <mark.clements at ki.se<mailto:mark.clements at ki.se>> wrote:
Adam: thank you for your prompt reply.

To define defaultSettings properly, is there some way to constraint the
type union (keep ++ change) to an existing type (e.g. {A:int, B:int})?
The following also do not work:

fun defaultSetting
     [keep] [change] [keep ~ change]
     (args : $change)
     : $(keep ++ change) =
     update {A=1, B=1} args

(* or *)

fun defaultSetting
         [keep] [change] [keep ~ change]
         (args : $change)
     : $(keep ++ change) =
     {A=1,B=1} --- change ++ args

-- Mark

On 29/10/19 12:52 am, Adam Chlipala wrote:
> On 10/28/19 3:27 PM, Mark Clements wrote:
>> fun defaultSetting args =
>>       update {A=1, B=1} args
>>
>> val _ = defaultSetting {A=2} (* {A=2, B=1} *)
>> val _ = defaultSetting {B=2} (* {A=1, B=2} *)
> The problem is that polymorphism in Ur/Web is always declared
> explicitly, not inferred as OCaml or Haskell.  You must add binders
> for type variables for defaultSettings just as you did for update.
> (This statement applies to definitions of functions, not uses, so the
> two example calls above should work once you get defaultSettings
> defined properly.)



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