[Ur] using mapX

[Sergey Mironov]

Thanks! The {Unit} - style code works!

Sergey

2013/10/2 Adam Chlipala <adamc at csail.mit.edu>:
> On 10/02/2013 08:41 AM, Adam Chlipala wrote:
>>
>> On 10/02/2013 06:28 AM, Sergey Mironov wrote:
>>>
>>> I attempted to declare xmlify like that:
>>>
>>> fun xmlify
>>>    [ctx ::: {Unit}]
>>>    [t ::: {Type}]
>>>    (fl : folder (map (fn _ =>  xml ctx [] []) t))
>>>    (r : record (map (fn _ =>  xml ctx [] []) t))
>>>       : xml ctx [] [] =
>>>      @mapX [ident] [ctx]
>>>        (fn [nm ::_] [t ::_] [r ::_] [[nm]~r] x =>  x)
>>>          fl r
>>
>>
>> The [ident] above indicates the type-level identity function, which
>> doesn't mesh with the type you chose for [r], and which explains why the
>> compiler expects [x] to have type [t].  Instead of [ident], you should write
>> [fn _ => xml ctx [] []] again.
>>
>> By the way, you'll probably find use of this function easier if you change
>> the kind of [t] to [{Unit}].  Since you're ignoring the types stored inside
>> [t] at present, type inference won't be able to reconstruct them uniquely
>> from context.
>
>
> Oh, and to make that more concrete, if I replace [{Type}] with [{Unit}],
> then your example works with one small type annotation to guide the choice
> of [ctx]:
>     val all : xbody = xmlify { A = <xml><p>blabla</p></xml> , B =
> <xml><p>zzz</p></xml> }
>
> Without that change, undetermined unification variables remain.