robertk@tekig.UUCP (Robert Kaires) (02/07/85)
Do any of you FORTH experts out there know the answer to this: How do you redefine the "+" function (or any other operator) such that entering "2 + 2" yields "4" instead of typing "2 2 +". Is this impossible in FORTH ? thanks
jhv@houxu.UUCP (James Van Ornum) (02/08/85)
Michael Stolowitz presented the paper "Algebraic Expression Evaluation in
FORTH" at a Computer Faire (I don't know which year - just that it was 1982
or later) with the following screens to convert infix notation to
postfix for evaluation:
0 ( ALGEBRAIC )
1 CREATE OP 44 ALLOT
2
3 : ?INTERP ( pfa -- ) CFA STATE @ IF , ELSE EXECUTE THEN ;
4
5 : OPP@ ( -- addr ) OP DUP @ + ;
6
7 : >OP ( pfa lev -- ) 4 OP +! OPP@ 21 ;
8
9 : OP> ( -- ) OPP@ 2@ -4 OP +! DROP ?INTERP ;
10
11 : LEV? ( -- lev ) OPP@ @ ;
12
13 : ]A BEGIN LEV? WHILE OP> REPEAT
14 [COMPILE] FORTH ; IMMEDIATE
15 -->
0 ( ALGEBRAIC )
1 : INFIX ( lev -- ) ( old rpn op new infix op )
2 ' CREATE SWAP , , IMMEDIATE
3 DOES> 2@ BEGIN DUP LEV? > NOT WHILE
4 >R >R OP> R> R> REPEAT >OP ;
5
6 VOCABULARY ALGEBRAIC IMMEDIATE ALGEBRAIC DEFINITIONS
7
8 7 INFIX * * 7 INFIX / /
9 6 INFIX + + 6 INFIX - -
10 5 INFIX > > 5 INFIX < < 5 INFIX = =
11 4 INFIX NOT NOT
12 3 INFIX AND AND
13 2 INFIX OR OR
14
15 -->
0 ( ALGEBRAIC )
1 : ( ['] CR 1 >OP ; IMMEDIATE
2
3 : ) FORTH BEGIN 1 LEV? < WHILE OP> REPEAT
4 1 LEV? = IF -4 OP +!
5 ELSE 1 ABORT" Missing (" THEN ; IMMEDIATE
6
7 FORTH DEFINITIONS
8
9 : A[ 0 OP ! [COMPILE] ALGEBRAIC ; IMMEDIATE EXIT
10
11 Examples: A[ A + B - C * ( D / A ) ]A
12
13 or : EXPR A[ A + B - C * ( D / A ) ]A ;
14
15
-----------------------
James Van Ornum, AT&T Bell Laboratories, inhp4!houxu!jhvmatheus@uiucdcsb.UUCP (02/08/85)
"impossible" is not a defined FORTH word!
breuel@harvard.ARPA (Thomas M. Breuel) (02/09/85)
> Do any of you FORTH experts out there know the answer to this: > How do you redefine the "+" function (or any other operator) such that > entering "2 + 2" yields "4" instead of typing "2 2 +". Is this > impossible in FORTH ? > thanks To make it work interpreted, you have to build some kind of interpreter for BASIC like expressions. I don't think that that's worth it. To make it work compiled, you have to write a parser which translates infix to postfix notation. The easiest way of doing that would probably be to have a compiler word, say 'let', and have it do the parsing, e.g. : ComputeIt let x = a * 9 + 3 ; ; I believe, though, that this is hardly useful and very confusing. In particular, if you don't have a mechanism for accessing local variables by name, then computations involving data on the stack are very hard to do. Thomas.
dgary@ecsvax.UUCP (D Gary Grady) (02/11/85)
> Do any of you FORTH experts out there know the answer to this: > How do you redefine the "+" function (or any other operator) such that > entering "2 + 2" yields "4" instead of typing "2 2 +". Is this > impossible in FORTH ? You can do something like this: : + 32 WORD NUMBER + ; This defines plus to be a word that reads in the following word, converts it to a number on the stack, then adds that to the number previously put there. This is almost identical to a similar word defined on page 277 of Brodie's _Starting Forth_. A similar idea is used with almost all text-oriented operations in Forth. Although Forth uses postfix notation for numbers, for text it (of practical necessity) reverts to prefix or infix. -- D Gary Grady Duke U Comp Center, Durham, NC 27706 (919) 684-3695 USENET: {seismo,decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgary
dpa@snow.UUCP (David Angier) (02/14/85)
For interpretive infix use of '+' you could use WORD to get the next number from the input stream and then use NUMBER to push the number on the stack, then add them together using the old version of '+'. Since you can check whether you are compiling or not you can make this safe, also you could make it use WORD to get the next word then NUMBER and finally LITERAL to compile it in RPN, afterwards use COMPILE + to put the old + in. I don't think it is worth it, but it can be done. I once saw a Pascal compiler that was writeen in Forth (UGH). Dave (Maths @ University of Warwick,UK)