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{---------------------------------------------------------------}
{ Recognize and Translate an IF Construct }
procedure Block; Forward;
procedure DoIf;
var L1, L2: string;
begin
Match('i');
BoolExpression;
L1 := NewLabel;
L2 := L1;
BranchFalse(L1);
Block;
if Look = 'l' then begin
Match('l');
L2 := NewLabel;
Branch(L2);
PostLabel(L1);
Block;
end;
PostLabel(L2);
Match('e');
end;
{--------------------------------------------------------------}
{ Parse and Translate a WHILE Statement }A62A6
- 29 -A*2A*
PA 2A
procedure DoWhile;
var L1, L2: string;
begin
Match('w');
L1 := NewLabel;
L2 := NewLabel;
PostLabel(L1);
BoolExpression;
BranchFalse(L2);
Block;
Match('e');
Branch(L1);
PostLabel(L2);
end;
{--------------------------------------------------------------}
To tie everything together, we need only modify procedure Block
to recognize the "keywords" for the IF and WHILE. As usual, we
expand the definition of a block like so:
<block> ::= ( <statement> )*
where
<statement> ::= <if> | <while> | <assignment>
The corresponding code is:
{--------------------------------------------------------------}
{ Parse and Translate a Block of Statements }
procedure Block;
begin
while not(Look in ['e', 'l']) do begin
case Look of
'i': DoIf;
'w': DoWhile;
else Assignment;
end;
end;
end;
{--------------------------------------------------------------}
OK, add the routines I've given, compile and test them. You
should be able to parse the single-character versions of any of
the control constructs. It's looking pretty good!A62A6
- 30 -A*2A*
PA 2A
As a matter of fact, except for the single-character limitation
we've got a virtually complete version of TINY. I call it, with
tongue planted firmly in cheek, TINY Version 0.1.
LEXICAL SCANNING
Of course, you know what's next: We have to convert the program
so that it can deal with multi-character keywords, newlines, and
whitespace. We have just gone through all that in Part VII.
We'll use the distributed scanner technique that I showed you in
that installment. The actual implementation is a little
different because the way I'm handling newlines is different.
To begin with, let's simply allow for whitespace. This involves
only adding calls to SkipWhite at the end of the three routines,
GetName, GetNum, and Match. A call to SkipWhite in Init primes
the pump in case there are leading spaces.
Next, we need to deal with newlines. This is really a two-step
process, since the treatment of the newlines with single-
character tokens is different from that for multi-character ones.
We can eliminate some work by doing both steps at once, but I
feel safer taking things one step at a time.
Insert the new procedure:
{--------------------------------------------------------------}
{ Skip Over an End-of-Line }
procedure NewLine;
begin
while Look = CR do begin
GetChar;
if Look = LF then GetChar;
SkipWhite;
end;
end;
{--------------------------------------------------------------}
Note that we have seen this procedure before in the form of
Procedure Fin. I've changed the name since this new one seems
more descriptive of the actual function. I've also changed the
code to allow for multiple newlines and lines with nothing but
white space.
The next step is to insert calls to NewLine wherever we decide a
newline is permissible. As I've pointed out before, this can be
very different in different languages. In TINY, I've decided to
allow them virtually anywhere. This means that we need calls to
NewLine at the BEGINNING (not the end, as with SkipWhite) of the
procedures GetName, GetNum, and Match.A*2A*
- 31 -
PA 2A
For procedures that have while loops, such as TopDecl, we need a
call to NewLine at the beginning of the procedure AND at the
bottom of each loop. That way, we can be assured that NewLine
has just been called at the beginning of each pass through the
loop.
If you've got all this done, try the program out and verify that
it will indeed handle white space and newlines.
If it does, then we're ready to deal with multi-character tokens
and keywords. To begin, add the additional declarations (copied
almost verbatim from Part VII):
{--------------------------------------------------------------}
{ Type Declarations }
type Symbol = string[8];
SymTab = array[1..1000] of Symbol;
TabPtr = ^SymTab;
{--------------------------------------------------------------}
{ Variable Declarations }
var Look : char; { Lookahead Character }
Token: char; { Encoded Token }
Value: string[16]; { Unencoded Token }
ST: Array['A'..'Z'] of char;
{--------------------------------------------------------------}
{ Definition of Keywords and Token Types }
const NKW = 9;
NKW1 = 10;
const KWlist: array[1..NKW] of Symbol =
('IF', 'ELSE', 'ENDIF', 'WHILE', 'ENDWHILE',
'VAR', 'BEGIN', 'END', 'PROGRAM');
const KWcode: string[NKW1] = 'xilewevbep';
{--------------------------------------------------------------}
Next, add the three procedures, also from Part VII:
{--------------------------------------------------------------}
{ Table Lookup }
function Lookup(T: TabPtr; s: string; n: integer): integer;A*2A*
- 32 -
PA 2A
var i: integer;
found: Boolean;
begin
found := false;
i := n;
while (i > 0) and not found do
if s = T^[i] then
found := true
else
dec(i);
Lookup := i;
end;
{--------------------------------------------------------------}
.
.
{--------------------------------------------------------------}
{ Get an Identifier and Scan it for Keywords }
procedure Scan;
begin
GetName;
Token := KWcode[Lookup(Addr(KWlist), Value, NKW) + 1];
end;
{--------------------------------------------------------------}
.
.
{--------------------------------------------------------------}
{ Match a Specific Input String }
procedure MatchString(x: string);
begin
if Value <> x then Expected('''' + x + '''');
end;
{--------------------------------------------------------------}
Now, we have to make a fairly large number of subtle changes to
the remaining procedures. First, we must change the function
GetName to a procedure, again as we did in Part VII:
{--------------------------------------------------------------}
{ Get an Identifier }
procedure GetName;
begin
NewLine;
if not IsAlpha(Look) then Expected('Name');
Value := '';
while IsAlNum(Look) do begin
Value := Value + UpCase(Look);
GetChar;
end;
SkipWhite;A*2A*
- 33 -
PA 2A
end;
{--------------------------------------------------------------}
Note that this procedure leaves its result in the global string
Value.
Next, we have to change every reference to GetName to reflect its
new form. These occur in Factor, Assignment, and Decl:
{---------------------------------------------------------------}
{ Parse and Translate a Math Factor }
procedure BoolExpression; Forward;
procedure Factor;
begin
if Look = '(' then begin
Match('(');
BoolExpression;
Match(')');
end
else if IsAlpha(Look) then begin
GetName;
LoadVar(Value[1]);
end
else
LoadConst(GetNum);
end;
{--------------------------------------------------------------}
.
.
{--------------------------------------------------------------}
{ Parse and Translate an Assignment Statement }
procedure Assignment;
var Name: char;
begin
Name := Value[1];
Match('=');
BoolExpression;
Store(Name);
end;
{---------------------------------------------------------------}
.
.
{--------------------------------------------------------------}
{ Parse and Translate a Data Declaration }
procedure Decl;
begin
GetName;
Alloc(Value[1]);A*2A*
- 34 -
PA 2A
while Look = ',' do begin
Match(',');
GetName;
Alloc(Value[1]);
end;
end;
{--------------------------------------------------------------}
(Note that we're still only allowing single-character variable
names, so we take the easy way out here and simply use the first
character of the string.)
Finally, we must make the changes to use Token instead of Look as
the test character and to call Scan at the appropriate places.
Mostly, this involves deleting calls to Match, occasionally
replacing calls to Match by calls to MatchString, and Replacing
calls to NewLine by calls to Scan. Here are the affected
routines:
{---------------------------------------------------------------}
{ Recognize and Translate an IF Construct }
procedure Block; Forward;
procedure DoIf;
var L1, L2: string;
begin
BoolExpression;
L1 := NewLabel;
L2 := L1;
BranchFalse(L1);
Block;
if Token = 'l' then begin
L2 := NewLabel;
Branch(L2);
PostLabel(L1);
Block;
end;
PostLabel(L2);
MatchString('ENDIF');
end;
{--------------------------------------------------------------}
{ Parse and Translate a WHILE Statement }
procedure DoWhile;
var L1, L2: string;
begin
L1 := NewLabel;
L2 := NewLabel;
PostLabel(L1);A*2A*
- 35 -
PA 2A
BoolExpression;
BranchFalse(L2);
Block;
MatchString('ENDWHILE');
Branch(L1);
PostLabel(L2);
end;
{--------------------------------------------------------------}
{ Parse and Translate a Block of Statements }
procedure Block;
begin
Scan;
while not(Token in ['e', 'l']) do begin
case Token of
'i': DoIf;
'w': DoWhile;
else Assignment;
end;
Scan;
end;
end;
{--------------------------------------------------------------}
{ Parse and Translate Global Decl?? 快捷鍵說明
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