Background - I'm completely new to f#, and haven't done much with ML at all in the past, my student days are more than 10 years in the past, when I briefly dabbled with SML/NJ.

I'm using f# 1.9.1.9, and I've tried extending the parser example with an incr statement, to match the decr statement.

Apart from adding in the relevant changes to the lexer and parser definitions, I changed the interpreter code as follows

// Copyright (c) Microsoft Corporation 2005-2006.
// This sample code is provided "as is" without warranty of any kind.
// We disclaim all warranties, either express or implied, including the
// warranties of merchantability and fitness for a particular purpose.
//

#light

open Ast

type v = INT of int | FLOAT of float

let print_val os v =
match v with
| INT n -> Printf.fprintf os "%d" n
| FLOAT f -> Printf.fprintf os "%g" f

let rec prog (Prog l ) =
stmts (Hashtbl.create 2000) l

and stmts s l =
List.iter (stmt s) l

and stmt s st =
match st with
| Assign (a,b) ->
Hashtbl.add s a (expr s b)
| While (a,b) ->
while (expr s a <> INT 0) do
stmt s b
done
| Seq l ->
stmts s l
| IfThen (g,t) ->
if (expr s g <> INT 0)
then stmt s t
| IfThenElse (g,t,e) ->
if (expr s g <> INT 0)
then stmt s t
else stmt s e
| Print (e) ->
Printf.printf "--> %a\n" print_val (expr s e);
flush stdout

and expr s e =
match e with
| Val n ->
if Hashtbl.mem s n
then Hashtbl.find s n
else
Printf.eprintf "warning: location %s not defined\n" n;
INT 0
| Int n -> INT n
| Float f -> FLOAT f
| Decr e2 ->
match expr s e2 with
| INT n -> INT (n-1)
| FLOAT f -> failwith "cannot decrement a float"
| Incr e2 ->
match expr s e2 with
| INT n -> INT (n+1)
| FLOAT f -> failwith "cannot increment a float"

I altered the sample program as follows :-

a := 1;
b := 0;
if a then d := 20;
if b then d := 40;
c:=20;
print d;
while c do
begin
c := decr(c);
d := incr(d);
print d ;
end;
print d

The output is as follows, it appears that the value of d is being reset at random during execution.

Mystifying ( at least to myself)

C:\Program Files\FSharp-1.9.1.9\samples\fsharp\Parsing>interp.exe test2.lang
#stmts = 8
running program...
--> 20
--> 21
--> 22
--> 23
--> 24
--> 21
--> 22
--> 23
--> 24
--> 25
--> 26
--> 27
--> 28
--> 29
--> 25
--> 26
--> 27
--> 28
--> 29
--> 30
--> 31
--> 31