[Ur] runST monad in ur/web

[Artella Coding]

Hi Adam/Ziv, thanks for the information.

*"The short answer is to use the FFI to do the relevant computation in
another language. C is the easiest choice to get working, but Edward Yang's
Logitext sets up Haskell for this. You're then free to give the function a
non-transaction type."*

Is there an example (in C) of this on the webpage or github (I could only
find documentation but no specific example).

For example suppose I have an array of ints in ur. I pass this to a c
function

int addArray(int* p, int sizeOfArray)

and then get the sum of the elements of the array back in ur/web. And then
I wrap the operation as a pure function (i.e. non-transaction type).

Thanks

On Tue, Dec 29, 2015 at 5:48 PM, Ziv Scully <ziv at mit.edu> wrote:

> The short answer is to use the FFI to do the relevant computation in
> another language. C is the easiest choice to get working, but Edward Yang's
> Logitext sets up Haskell for this. You're then free to give the function a
> non-transaction type.
>
> It might be possible to write the ST monad using the FFI. One can
> implement types and functions for references, arrays, etc. in C, use .urp
> declarations mark all these functions as effectful, and wrap these types
> and functions in a module that enforces the typical ST types, which is
> straightforward with Ur/Web's rank-N types. That said, the lack of garbage
> collection could make this complicated. I'm not sure how safe it is to
> store Ur/Web structures in references (the structure might get freed) or
> how to free any allocated ST structures. The first problem has an easy
> half-solution: if there are some types that are safely usable in ST
> references, use a closed type class to enumerate them. (See the interface
> for Basis.sql_injectable for a similar example.)
>
>
> On Tuesday, December 29, 2015, Adam Chlipala <adamc at csail.mit.edu> wrote:
>
>> I wonder if this source file would be of interest, from Sergey Mironov's
>> urweb-monad-pack library.
>>     https://github.com/grwlf/urweb-monad-pack/blob/master/state.ur
>>
>> I'm not familiar enough with Haskell myself to know whether it's a
>> perfect match.
>>
>> On 12/23/2015 11:16 AM, Artella Coding wrote:
>>
>>> Hi, is there something like a haskell runST monad (
>>> https://wiki.haskell.org/Monad/ST) in ur/web, so that I can perform
>>> mutations within a pure function?
>>>
>>> For example I might want to have my pure function make a copy of a large
>>> array, and then perform the relevant mutations on this copy, before
>>> returning this copy. The function is pure in that it does not modify the
>>> input nor does it require data from the "outside world", and yet relies on
>>> mutations of the internal copy (which it employs for the sake of
>>> efficiency).
>>>
>>
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