Beginner's question: open and close file

[Shimin Guo]

I wrote the following code

#include "share/atspre_define.hats"

#include "share/atspre_staload.hats"

staload "libats/libc/SATS/stdio.sats"

fun open_close(): void =

let

  val fptr = fopen("hello.dats", file_mode_r)

  val p = ptrcast(fptr)

in

  if p = 0

    then

      let

        prval () = FILEptr_free_null(fptr)

      in

      end

    else

      let

        val (pf|rc) = fclose(fptr)

        val () = assertloc(rc = 0) // fclose is expected to succeed

        prval None_v() = pf

      in

      end

end

and got the following compiler error:

/home/sguo/ats/file.dats: 242(line=13, offs=7) -- 307(line=16, offs=10): error(3): the dynamic variable [fptr$4718(-1)] is consumed but it should be retained with the type [S2Eapp(S2Ecst(ptr_addr_type); S2Evar(l$8617$8618(14269)))] instead.

patsopt(TRANS3): there are [1] errors in total.

exit(ATS): uncaught exception: _2home_2hwxi_2Research_2ATS_2dPostiats_2src_2pats_error_2esats__FatalErrorExn(1025)

Why does it say fptr should be retained?

[Hongwei Xi]

Here is a quick fix:

extern
praxi
clear{a:t0ype}(x: INV(a)): void

fun open_close(): void =
let
  val fptr = fopen("hello.dats", file_mode_r)
  val p = ptrcast(fptr)
in
  if p = 0
    then
      let
        prval () = FILEptr_free_null(fptr)
      in
      end
    else
      let
        val (pf|rc) = fclose(fptr)
        val () = assertloc(rc = 0) // fclose is expected to succeed
        prval None_v() = pf

        prval () = clear(fptr)
      in
      end
end

######

The above shows a C-style of dealing with IO in ATS. What I like to recommend

is to deal with IO via the use of linear streams:

https://github.com/ats-lang/ATS-CodeBook/tree/master/RECIPE/ReadFromSTDIN

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[Vanessa McHale]

FWIW, I personally would use

FILEptr_is_null to check if fptr is null, rather than ptrcast and then a comparison to 0

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[Shimin Guo]

Thanks for the tip! Will look into linear streams. 

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[Shimin Guo]

Cool. I was looking in http://ats-lang.github.io/DOCUMENT/ATS-Postiats/libats/libc/HTML/ATSLIB_libats_libc_all_in_one.html (linked from http://www.ats-lang.org/Libraries.html) and FILEptr_is_null is not there. But it is in http://ats-lang.sourceforge.net/DOCUMENT/ATS-Postiats/libats/libc/SATS/stdio.sats.

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[Shimin Guo]

On Tue, Mar 12, 2019 at 12:23 PM Hongwei Xi <gmh...@gmail.com> wrote:

I'm trying to implement tail in ATS as a practice. It seems the streams interface may not be a good fit in this case. Is there a better way to do it than the C-style way? (read the last N bytes into a buffer, count the number of new lines from the end) 

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[Hongwei Xi]

You can use a stream of chars for the content of a given file.

Or use a stream of blocks of chars for the content of a given file,

where each block is of size, say, 4096 bytes. For the start, using

a stream of chars is a good choice. Please stay away from the urge

to write "efficient" code at this stage.

By using streams, you can readily do stream-processing in parallel:

http://ats-lang.sourceforge.net/EXAMPLE/EFFECTIVATS/StreamPar/.

This is something that you would not easily get when coding in C-style.

Of course, one could implement a parallel version of 'tail' but very few

people would give it a try due to the high programming cost. Economically

speaking, the very goal of ATS is to greatly reduce programming cost and increase

programming productivity. My hope is to use ATS3 to give concrete demonstration

convincingly.

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