Re: [fstar-club] Trying to use List.Tot.map with Kremlin

[Clément Pit-Claudel via fstar-club]

On 2018-05-02 12:08, Jonathan Protzenko wrote:
> I generally use meta-programming to refer to any partial program
> evaluation that is performed by F* at type-checking time instead of
> being performed at run-time, but perhaps that's an incorrect term...
> any suggestions for a better term? 

I usually just call it partial evaluation, as you did above :)
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Re: [fstar-club] Trying to use List.Tot.map with Kremlin

[Clément Pit-Claudel via fstar-club]

Hmm, I don't think this involves meta-programming; normalize_term is a pure 
function with special support in the compiler.

The meta-programming version would look like this, rather:

open FStar.Tactics

let main () : int =
  C.print_string (C.string_of_literal (synth_by_tactic (fun () ->
  let body = `(String.concat ", " ["hello"; "world"]) in
  exact (norm_term [primops; delta; iota; zeta] body;
  C.exit_success

Clément.


On 2018-05-02 11:47, Jonathan Protzenko wrote:
> Yes -- you can always meta-program as much as you want, although:
> 
>   * meta-programming support is limited for string functions -- I think only 
> String.length can be meta-evaluated
>   * the chapter on meta-programming Low* hasn't been written yet in the 
> tutorial .
> 
> 
> ~ jonathan
> 
> --
> *From:* fstar-club <fstar-club-boun...@lists.gforge.inria.fr> on behalf of 
> Clément Pit-Claudel via fstar-club <fstar-club@lists.gforge.inria.fr>
> *Sent:* Wednesday, May 2, 2018 8:44 AM
> *To:* fstar-club@lists.gforge.inria.fr
> *Subject:* Re: [fstar-club] Trying to use List.Tot.map with Kremlin
>  
> I'd expect the following to work, though. Does it?
> 
>  let main () =
>    C.print_string (C.string_of_literal (normalize_term (String.concat ", 
> " ["hello"; "world"])));
>    C.exit_success
> 
> … though maybe we don't apply `normalize_term` early enough.
> 
> Clément.
> 
> On 2018-05-02 11:08, Jonathan Protzenko via fstar-club wrote:
>> Yes, this is something that has poor usability. Prims.string and 
>> FStar.String.concat are supported as transition mechanisms to help porting 
>> F* programs to Low* but I agree that it's confusing for beginners to have 
>> them work "by default".
>> 
>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffstarlang.github.io%2Flowstar%2Fhtml%2FLowStar.html%23c-string-literals=02%7C01%7Cprotz%40microsoft.com%7C6270df4eb0a244eaac3908d5b043b2d5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636608727210614165=ffbZwtJJ2ud7JXMhD56SL3Kyv15X%2BEYQT87%2Bc%2BlLFgY%3D=0
>>  has a note that explains it, my plan is to make it a warning that's fatal 
>> by default, and that can be disabled if you need these mechanisms
>> 
>> ~ jonathan
>> 
>> --
>> *From:* Pierre Beaucamp <m...@pierrebeaucamp.com>
>> *Sent:* Tuesday, May 1, 2018 4:38 PM
>> *To:* Jonathan Protzenko; Pierre Beaucamp via fstar-club
>> *Subject:* Re: [fstar-club] Trying to use List.Tot.map with Kremlin
>>  
>> Thanks for the quick answer Jonathan.
>> 
>> I already assumed that this is caused by limited inter-polarity between F* 
>> strings and their representation in Low*. What threw me off is that this 
>> code works:
>> 
>>     let main () =
>>    

Re: [fstar-club] Trying to use List.Tot.map with Kremlin

[Clément Pit-Claudel via fstar-club]

I'd expect the following to work, though. Does it?

 let main () =
   C.print_string (C.string_of_literal (normalize_term (String.concat ", " 
["hello"; "world"])));
   C.exit_success

… though maybe we don't apply `normalize_term` early enough.

Clément.

On 2018-05-02 11:08, Jonathan Protzenko via fstar-club wrote:
> Yes, this is something that has poor usability. Prims.string and 
> FStar.String.concat are supported as transition mechanisms to help porting F* 
> programs to Low* but I agree that it's confusing for beginners to have them 
> work "by default".
> 
> https://fstarlang.github.io/lowstar/html/LowStar.html#c-string-literals has a 
> note that explains it, my plan is to make it a warning that's fatal by 
> default, and that can be disabled if you need these mechanisms
> 
> ~ jonathan
> 
> --
> *From:* Pierre Beaucamp 
> *Sent:* Tuesday, May 1, 2018 4:38 PM
> *To:* Jonathan Protzenko; Pierre Beaucamp via fstar-club
> *Subject:* Re: [fstar-club] Trying to use List.Tot.map with Kremlin
>  
> Thanks for the quick answer Jonathan.
> 
> I already assumed that this is caused by limited inter-polarity between F* 
> strings and their representation in Low*. What threw me off is that this code 
> works:
> 
>     let main () =
>   C.print_string (C.string_of_literal (String.concat ", " ["hello"; 
> "world"]));
>   C.exit_success
> 
> Either way, I'm excited for the Low* tutorial - thank you for writing it. 
> This is a very interesting language with unfortunately very little 
> documentation for new-comers (I'm pretty much trying to learn F* and Low* at 
> the same time, and I guess this is where a lot of the confusion is coming 
> from).
> 
> Thank you and best regards,
> 
> -- 
> Pierre Beaucamp
> 
> On Tue, May 1, 2018, at 7:09 PM, Jonathan Protzenko wrote:
>> Hello Pierre,
>> 
>> These are all excellent questions -- I am currently writing a tutorial 
>> for Low* are these served as good inspiration for the introduction. If 
>> CI goes through, a work-in-progress version of the tutorial should be 
>> uploaded in the next few hours at 
>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffstarlang.github.io%2Flowstar%2F=02%7C01%7Cprotz%40microsoft.com%7Cdb40113318a44ce8c0be08d5afbc9c55%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636608146970557404=aLvfTn4cd9Wrvck8prnVvnQfEA9I3BPpp1%2BE0PkjGpw%3D=0
>> -- please read it and let me know whether after browsing it, your 
>> questions have been answered.
>> 
>> In case the CI doesn't perform the upload, or the tutorial is unclear, 
>> the essence of your issue is that your code is not Low*.
>> 
>>   *   the F* string and list types are F* values, i.e. concat has type 
>> string -> string -> string; the only way to give you these semantics 
>> without requiring the user to track allocations and lifetimes is to use 
>> a garbage-collector, which C does not have
>>   *   the same goes for lists: compiling them to C with the current F* 
>> value semantics would lead to a zillion uncollected list cell 
>> allocations.
>> 
>> What the C.String module provides is a bare-bones model of C string 
>> literals, i.e. `const char *s = "my string literal";`, meaning that you 
>> should provide an actual literal, e.g. `C.String.( print !$"hello, world
>> \n" );`. This is very limited, and does not allow you to do arbitrary 
>> string manipulation.
>> 
>> A proof-of-concept example exists that does that, in the test/ 
>> subdirectory of KreMLin, and is called Server.fst. It models blitting 
>> C.String.t's into a mutable buffer of C chars, tracking how much space 
>> remains in your buffer, whether the buffer is live, etc. and rules out 
>> buffer overflows.
>> 
>> Naturally, it's a little involved for a hello world, but I guess that's 
>> the reality of tracking temporal (liveness) and spatial safety (bounds) 
>> in C.
>> 
>> Let me know if that helps.
>> 
>> Cheers,
>> 
>> Jonathan
>> 
>> 
>> ~ jonathan
>> 
>>