I wish to be able to quit a program in the middle of a procedure which
normally accepts only numerical values. I would like the procedure to accept
the letter Q as well as a number. I have the following source code  -

Var Number: Real;
Repeat
   Write(' Enter a number or Q to quit .....  ');
   {$I-}   Read(Number)    {$I+}
Until IOresult=0

Program ExitOnQ;
{Turbo v6.0}

VAR
exitFlag:Boolean;

Procedure EnterData(flg:Boolean);
Var Number: Real;
    Entry:String;
    err:Integer;
Begin
     Repeat
           Write('Enter a number or Q to quit ..... ');
           Readln(Entry);
           flg := Upcase(Entry[1]) = 'Q';
           If not flg then VAL(Entry, Number, err);
           If err = 0 then Writeln(Number) else Writeln('Try again');
     Until flg;
End;

BEGIN
     Writeln; Writeln;
     EnterData(exitFlag);
     If exitFlag then Exit;
END.

Personally I would never use the  >    {$I-}   Read(Number)    {$I+}
and ioresult to decide whether you should quit or not. It means that any
mistype would lead to quitting even if the user of the program did not
intend to.

The way I solved this is
1 I wrote a little editor function that edits strings
    function editstring(s:string):string;
    arrow left/right = move cursor
    esc exits, the function returns the unmodified string
    enter accepts and the funtion returns the edited string
2 Two funtions (for the integer type and for the real type)
   the function passes the value of the parameter to the editstring
function as a string. The returned string is analysed with VAL. If the
returned string can't be interpreted the value of the parameter remains
unchanged.

here's some code. It's ripped from a larger package. But it gives an
idea.
You need a mouse unit for it, or strip all the mouse commands from the
code.

--Femme
-----------------------------------------
Program testedit;

uses crt,mouse;

Procedure piep;
begin
  sound(2000);
  delay(100);
  nosound;
end;

function decimals(x:extended;signif:integer):integer;
   {write(myreal:0:decimals(myreal,3)
    returns the number with only 3 significant digits
    e.g. 1.23   or 0.000123 }
var d:integer;
begin
   x:=abs(x);
   if x<1e-10 then d:=-1 else d:=-round(ln(x)/ln(10)-0.4999)+signif-1;
   if d<0 then decimals:=0 else decimals:=d;
end;

function i2s(i:longint):string;  {integer to string conversion}
var s:string;
begin
  str(i,s);
  i2s:=s;
end;

function R2S(r:extended;displdeci:integer):string;    {real to string}
var s:string;
begin
  str(r:0:decimals(r,displdeci),s);
  R2S:=s;
end;

const overwritemode:boolean=false;

function editstring(s:string):string;
var i,x,y:integer;
    ch:char;
    eerstekeer:boolean;
    dum:string[2];
    mb:word;
    mx,my:integer;
begin
  x:=wherex;
  y:=wherey;
  i:=1;
  eerstekeer:=true;
  editstring:=s;
  repeat
    gotoxy(x,y);
    write(s);
    clreol;
    gotoxy(x+i-1,y);
    if overwritemode then
    begin
      if i>length(s) then s[i]:=' ';
      write(s[i]);
      gotoxy(x+i-1,y);
    end;
    if mouseavailable then
    begin
      mouse_cursor(true);                         {show muis cursor}
      repeat until mousebutton=0;                 {wait till not mouse
clicked}
    end;

    repeat until (keypressed or mouse_clicked($FF)); {wait for mouse or
key}
    if mouseavailable then mouse_cursor(false);   {hide muis cursor}

    if keypressed then ch:=readkey else
    begin
      mx:=integer(mouse_clicked_x) div 8 +1-lo(windmin);
      my:=integer(mouse_clicked_y) div 8 +1-hi(windmin);
      if my<>y then ch:=#13 else
      begin
        ch:=#1;
        if mx<x then i:=1 else
          if (mx>=x+length(s)) then i:=length(s)+1 else i:= mx-x+1;
      end;
    end;

     case ch of
      #1 :;
      #27: exit;
      #13:begin
            editstring:=s;
            writeln;
            exit;
          end;
      #8 :if i>1 then begin                {backspace}
                        delete(s,i-1,1);
                        dec(i);
                      end;
      #32..#254:begin
                  if eerstekeer then s:='';
                  if (i>length(s))
                    then s:=s+ch
                    else if overwritemode then s[i]:=ch
                                        else  insert(ch,s,i);
                  inc(i);
                end;
      #17: if i>1 then dec(i);                 {Touch <}
      #26: if i<=length(s) then inc(i);        {Touch >}
      #18: if i<=length(s) then delete(s,i,1); {Touch del}
      #0:begin
           ch:=readkey;
           case ch of
     {<}     #75: if i>1 then dec(i);
     {>}     #77: if i<=length(s) then inc(i);
     {del}   #83: if i<=length(s) then delete(s,i,1);
     {ins}   #82: overwritemode := not overwritemode;
              else piep;
           end
         end;
    end;
    eerstekeer:=false;
  until false;
end;

function editinteger(i:longint):longint;
var s:string;
    Iedit:longint;
    code:integer;
begin
  editinteger:=i;
  s:=editstring(i2s(i));
  val(s,Iedit,code);
  if (code=0)
     then editinteger:=iedit
     else writeln(' Can''t interpret ',S);
end;

function editreal(r:extended;displdeci:integer):extended;
var s:string;
    redit:extended;
    code:integer;
begin
  editreal:=r;
  str(r:0:decimals(r,displdeci+4),s);
  s:=editstring(R2S(r,displdeci));
  val(s,redit,code);
  if (code=0)
     then editreal:=redit
     else  writeln(' Can''t interpret ',S);
end;

const i:integer=123;
      r:double=456.789;
begin
  write('I=');
  i:=editinteger(i);
  writeln('I=',i);

  write('R=');
  R:=editreal(R,8);
  writeln('R=',r:0:6);

flg:= (entry<>'') and (Upcase(Entry[1]='Q');

flg := (entry<>'') OR (Upcase(entry[1]='Q');