Translate regress/noinc/loop_while

[master-q]

Fibonacci is a good example?

• http://www.ats-lang.org/Examples.html
• https://www.programiz.com/c-programming/examples/fibonacci-series

[master-q]

We should convert loop into recursion:

• https://github.com/githwxi/ATS-Postiats/wiki/loops
• https://groups.google.com/forum/#!searchin/ats-lang-users/loop$20for$20while$20recursion|sort:date/ats-lang-users/q_w3a3dkXAM/XHpxyV25tXcJ

[master-q]

int fib(int n) {
    int f0 = 0, f1 = 1;

    while (n-- > 0) {
        int tmp = f1;
        f1 = f0 + f1;
        f0 = tmp;
    };

    return f0;
}

int main() {
    return fib(10) - 55;
}

should be translated into:

#include "share/atspre_staload.hats"

fun fib (n: int): int =
  let
    var f0: int = 0
    var f1: int = 1
    fun fib_loop (n: int, f1: int, f0: int): (int, int, int) =
      if n > 0 then let
          var n = n - 1
          var tmp: int = f1
          var f1 = f0 + f1
          var f0 = tmp
        in
          fib_loop (n, f1, f0)
        end
      else
        (n, f1, f0)
    val (n, f1, f0) = fib_loop (n, f1, f0)
  in
    f0
  end

implement main () =
  let
  in
    fib(10) - 55
  end

[master-q]

There are choices:

1. Introduce {l,r}-value flag
2. Change any r-value to l-value after introducing, such as match tuple / function define

[master-q]

Example 2 is following:

#include "share/atspre_staload.hats"

fun fib (n: int): int =
  let
    var n: int = n
    var f0: int = 0
    var f1: int = 1
    fun fib_loop (n: int, f1: int, f0: int): (int, int, int) =
      let
        var n: int = n
        var f1: int = f1
        var f0: int = f0
      in
        if n > 0 then let
            val () = n := n - 1
            var tmp: int = f1
            val () = f1 := f0 + f1
            val () = f0 := tmp
          in
            fib_loop (n, f1, f0)
          end
        else
          (n, f1, f0)
      end
    val (n', f1', f0') = fib_loop (n, f1, f0)
    val () = n := n'
    val () = f1 := f1'
    val () = f0 := f0'
  in
    f0
  end

implement main () =
  let
  in
    fib(10) - 55
  end

[master-q]

Need to modify interpretStatementExp :: C.CStat -> State IEnv (A.Expression Pos) for let in end.

[master-q]

Supported at 1be1bdc4c2930ffc53f47fff3f2e37872e568cab.