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LI31.BAS
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DECLARE FUNCTION xMid$ (string1$, start1!, stop1!)
DECLARE FUNCTION xNot! (bool!)
DECLARE FUNCTION xStr$ (value!)
DECLARE FUNCTION xRange! (NUM!, lval!, uval!)
DECLARE FUNCTION lenPair! (pair$)
DECLARE FUNCTION errSig$ (eCode$)
DECLARE FUNCTION consPair$ (fst$, rest$)
DECLARE FUNCTION getDatype$ (typet$)
DECLARE FUNCTION opCheck$ (operator$)
DECLARE FUNCTION selPairE$ (pair$, selE!)
DECLARE FUNCTION fRead$ (void$)
DECLARE FUNCTION evPair$ (pair$)
DECLARE FUNCTION eForm$ (form$)
DECLARE FUNCTION evAtom$ (atom$)
DECLARE FUNCTION applyOp$ (lst$)
'
' Skele's LISP Interpreter v3.0
' copyright (c) Skeleville Software 2006 - 2007
' This program emulates some of the functions found in a real LISP
' interpreter and also directly imports some of the functions from QBasic.
' It was created as a learning tool for LISP and so may contain some bugs
' in the lisp syntax expressions areas or some functions and features may
' not be present. Also, there will be some unique features.
' Enjoy and modify to your heart's content only leaving the copyright intact
' to show where the program is coming from.
' Created:
' 16/8/06
' Last Modified:
' 26/8/06
' Function declarations (no subs) to ease reading and faccilitate
' conversion to another language. All functions used must be compatible
' with some other language.
'
'
' Main module. This initializes global variables and initiates the
' read-evaluate-print loop which is the major operation of LISP. There is
' also a facility for reading the required expression from the commmand line.
'
' Notes:
' 1/9/06: add application saving and loading.
' add command line options.
' add help features.
' add auto-indenting to read phase.
' modify system commands to prevent syntax clashes.
' 3/9/06: modify to add, access and update size info for list elements to
' speed the car function. But would have to restructure most of the
' existing routines to use and there could be incompatibilities. Therefore,
' a converter is also in order.
' 7/9/06: add form & evaluation saving to memory. Can probably be used to
' bypass the evaluation phase of the R-E-P loop.
' 9/9/06: modify to avoid using space limited commands that may lead to
' qbasic errors, (eg. function calls, variables, etc.) instead, use
' commands that limit calls (goto), make variable size hardware limited
' (streams), etc.
' optimize code to avoid unnecessary calls.
' bug found where a single pair of numbers would also make a decimal, which
' could lead to more bugs. Will have to currently avoid creating single
' pairs of pure numbers.
' 12/9/06: bug found where a + signed number can be taken for a normal atom.
'
' History:
' 1/9/06: provided space for extended system commands.
' 7/9/06: added pre load and save commands.
' added some data type constants.
'
' Global constants:
CONST HEAD = 1, TAIL = -1, EL$ = "()", LF$ = "NIL", LT$ = "T", OB$ = "("
CONST CB$ = ")", LP$ = "list", AP$ = "dot-pair", atmT$ = "atom"
CONST numT$ = "number", blnT$ = "boolean", procT$ = "procedure"
CONST strT$ = "string"
DIM SHARED varMem$
varMem$ = "((()) (()))"
cmdLn$ = COMMAND$
IF cmdLn$ <> "" THEN
'arguments present on the command line. modify to check input.
PRINT eForm$(cmdLn$)
ELSE
CLS
PRINT
PRINT "Skele's LISP Interpreter v3.0"
PRINT "created by Skele Drew"
PRINT "copyright (c) Skeleville Software 2006 - 2007"
PRINT
PRINT "Current date and time is: "; DATE$; " "; TIME$
PRINT
DO
'interpreter read-eval-print loop. mod...
REPLoop:
'for debugging interpreter errors.
''ON ERROR GOTO ErrMan
PRINT
''expr$ = fRead$(void$)
INPUT expr$
SELECT CASE expr$
'check for any interpreter commands.
CASE "sys:exit"
'exit program.
PRINT
PRINT "Terminating Skele's LISP Interpreter!"
END
CASE "sys:pause"
'pause execution for debugging or other stuff.
STOP
CASE "sys:help"
'provide general help.
PRINT "Feature under construction"
CASE "sys:clear", " "
'clear the screen.
CLS
CASE "sys:time"
'print time and date.
PRINT "Current date and time is: "; DATE$; " "; TIME$
CASE ""
'nothing given.
STOP
CASE "sys:reset"
'clear and reinitialize program.
varMem$ = "((()) (()))"
PRINT "Interpreter memory reset!"
CASE "sys:save"
'write memory, forms and results to files.
memh = FREEFILE
OPEN "a:\memfile.rec" FOR OUTPUT AS memh
PRINT #memh, varMem$
CLOSE memh
CASE "sys:load"
'load memory file.
memh = FREEFILE
OPEN "a:\memfile.rec" FOR INPUT AS memh
LINE INPUT #memh, memstring$
'merge loaded memory contents with current memory.
CASE ELSE
'not a system command.
retval$ = eForm$(expr$)
END SELECT
IF errID <> 0 THEN
'print result and any errors.
retval$ = retval$ + CHR$(13) + CHR$(10) + errMsg$
END IF
PRINT
PRINT retval$
LOOP
END IF
ErrMan:
STOP
PRINT "Error in interpreter coded"; ERR
RESUME REPLoop
'obsolete scrap code:
IF expr$ = "exit" THEN
'exit program
PRINT
PRINT "Goodbye!"
END
END IF
FUNCTION applyOp$ (list$)
'applies an operator to its operands.
' Created:
' 26/8/06
' Last Modified:
' 3/9/06
'
' Input:
' application list.
' Output:
' application result.
'
' Notes:
' 26/8/06: largs$ - list arguments.
' lOpr$ - list operator.
' retval - result of mathematical procedures.
' modify math cases to check numbers and use long working methods.
' to upgrade all retro version cases.
' 2/9/06: after fixing a variable creation bug, it was noted that several
' bugs had been caused by uninitialized variables.
' 3/9/06: modify array-using routines to use the speed selector instead.
' 7/9/06: to add essential predicate procedures.
' to add looping forms.
' 9/9/06: modify procedure?
'
' History:
' 27/8/06: corrected minor bug.
' 2/9/06: corrected bug preventing variable creation.
' 3/9/06: added if conditional.
' corrected bugs in set operator.
' 5/9/06: corrected setq operator bugs.
' 9/9/06: fixed variable modification bug in set.
' added lambda procedure.
'
''lArgs$ = OB$+selPairE$(list$, -1)+CB$
''lOpr$ = selPairE$(list$, 1)
lstLen = VAL(selPairE$(list$, 0))
eleInd = lstLen - 1
DIM opList$(eleInd)
tmpInd = 0
dvarMem$ = EL$
DO
'store elements in array. ok.
opList$(tmpInd) = selPairE$(list$, tmpInd + 1)
tmpInd = tmpInd + 1
IF tmpInd > eleInd THEN
'finished creating array.
EXIT DO
END IF
LOOP
SELECT CASE opList$(0)
CASE "eval"
'evaluate argument.
applyOp$ = eForm$(opList$(1))
CASE "+"
'add stuff. ok.
retval = 0
FOR inc = 0 TO eleInd
'reference elements.
retval = retval + VAL(opList$(inc))
NEXT
applyOp$ = xStr$(retval)
CASE "+"
'add stuff. test. discarded for retro method (less work).
retval = 0
DO
'reference elements.
retval = retval + VAL(selPairE$(largs$, 1))
IF largs$ = EL$ THEN
'nothing more to add.
applyOp$ = xStr$(retval)
EXIT DO
END IF
largs$ = OB$ + selPairE$(largs$, -1) + CB$
LOOP
CASE "-"
'subtract stuff. ok
IF eleInd = 0 THEN
'no args.
retval = 0
ELSE
retval = VAL(opList$(1))
IF eleInd > 1 THEN
'more than 1 args present.
FOR inc = 2 TO eleInd
'reference args.
retval = retval - VAL(opList$(inc))
NEXT
END IF
END IF
applyOp$ = xStr$(retval)
CASE "*"
'multiply stuff. ok
retval = 1
IF eleInd > 0 THEN
'args present.
FOR inc = 1 TO eleInd
retval = retval * VAL(opList$(inc))
NEXT
END IF
applyOp$ = xStr$(retval)
CASE "/"
'divide stuff. ok.
IF eleInd > 1 THEN
'at least 2 args.
IF VAL(opList$(1)) = 0 THEN
'dividing 0.
retval = 0
ELSE
retval = VAL(opList$(1))
FOR inc = 2 TO eleInd
'divide args.
IF VAL(opList$(inc)) = 0 THEN
'division by 0!
void$ = errSig$("dbz")
errMsg$ = "Division by 0!"
EXIT FOR
ELSE
retval = retval / VAL(opList$(inc))
END IF
NEXT
END IF
ELSE
IF eleInd = 1 THEN
'single arg.
retval = 1 / VAL(opList$(1))
ELSE
'no args.
retval = 0
END IF
END IF
applyOp$ = xStr$(retval)
CASE "\", "rem"
'integer division. returns a list of result and remainder. ok.
'modify to return one of its two results.
IF eleInd > 1 THEN
'at least 2 args.
IF VAL(opList$(1)) = 0 THEN
'dividing 0.
retval = 0
ELSE
remval = VAL(opList$(1))
FOR inc = 2 TO eleInd
'divide args.
IF VAL(opList$(inc)) = 0 THEN
'division by 0!
void$ = errSig$("dbz")
errMsg$ = "Division by 0!"
EXIT FOR
ELSE
'special division by repeated subtraction.
IF inc > 2 THEN
'not first division (over 2 args.)
remval = retval
END IF
retval = 0
divval = VAL(opList$(inc))
WHILE remval >= divval
'subtract until remainder is less than divisor.
remval = remval - divval
retval = retval + 1
WEND
END IF
NEXT
END IF
ELSE
IF eleInd = 1 THEN
'single arg.
retval = 1 / VAL(opList$(1))
ELSE
'no args.
retval = 0
END IF
END IF
IF opList$(0) = "/" THEN
'return result.
applyOp$ = xStr$(retval)
ELSE
'return remainder.
applyOp$ = xStr$(remval)
END IF
CASE "x-power-y"
'finds one value to the power of another. mod...
applyOp$ = xStr$(VAL(opList$(1)) ^ VAL(opList$(2)))
CASE "x-root-y"
'finds the required root of a value.
applyOp$ = xStr$(VAL(opList$(1)) ^ (1 / (VAL(opList$(2)))))
CASE "abs"
'returns the absolute value.
applyOp$ = xStr$(ABS(VAL(opList$(1))))
CASE "cos"
'returns the cosine of an angle given in radians.
applyOp$ = xStr$(COS(VAL(opList$(1))))
CASE "tan"
'returns the tangent of an angle given in radians.
applyOp$ = xStr$(TAN(VAL(opList$(1))))
CASE "sin"
'returns the sine of a number given in radians.
applyOp$ = xStr$(SIN(VAL(opList$(1))))
CASE "arctan"
'returns the arc tangent.
applyOp$ = xStr$(ATN(VAL(opList$(1))))
CASE "<"
'ascending order (less than). ok.
retval = VAL(opList$(1))
FOR inc = 2 TO eleInd
'compare.
cmpval = VAL(opList$(inc))
IF retval < cmpval THEN
'in order check.
retval = cmpval
applyOp$ = LTRIM$(STR$(retval))
ELSE
applyOp$ = LF$
EXIT FOR
END IF
NEXT
CASE ">"
'descending order (greater than). ok.
retval = VAL(opList$(1))
FOR inc = 2 TO eleInd
'compare.
cmpval = VAL(opList$(inc))
IF retval > cmpval THEN
'in order check.
retval = cmpval
applyOp$ = LTRIM$(STR$(retval))
ELSE
applyOp$ = LF$
EXIT FOR
END IF
NEXT
CASE "and"
'evals up to nil value. ok.
IF eleInd = 0 THEN
'function only.
applyOp$ = clf$
ELSE
FOR inc = 1 TO eleInd
'eval args.
evalstr$ = eForm$(opList$(inc))
IF evalstr$ = clf$ THEN
'arg gives nil.
applyOp$ = clf$
EXIT FOR
ELSE
applyOp$ = evalstr$
END IF
NEXT
END IF
CASE "or"
'evals up to non-nil value. ok.
IF eleInd = 0 THEN
'function only.
applyOp$ = clf$
ELSE
FOR inc = 1 TO eleInd
'eval args.
evalstr$ = eForm$(opList$(inc))
IF evalstr$ = clf$ THEN
'arg gives nil.
applyOp$ = clf$
ELSE
applyOp$ = evalstr$
EXIT FOR
END IF
NEXT
END IF
CASE "car"
'return head element. ok.
IF getDatype$(opList$(1)) = LP$ THEN
'get first element only if list.
applyOp$ = selPairE$(opList$(1), 1)
ELSE
void$ = errSig$("lst")
errMsg$ = "Error: " + opList$(1) + " isn't of type list! Signalled by "
errMsg$ = errMsg$ + curFunc$ + " at level " + xStr$(level)
END IF
CASE "cdr"
'return any tail elements. ok.
IF getDatype$(opList$(1)) = LP$ THEN
'send to splitter only if list.
applyOp$ = OB$ + selPairE$(opList$(1), -1) + CB$
ELSE
void$ = errSig$("tp")
errMsg$ = "Error: " + opList$(1) + " isn't of type list! Signalled by "
errMsg$ = errMsg$ + curFunc$ + " at level" + xStr$(level)
END IF
CASE "cons"
'construct lists. ok.
applyOp$ = consPair$(opList$(1), opList$(2))
CASE "set", "setq"
'binds values to variables. ok?
IF opList$(0) = "setq" THEN
'convert set guote to set and evaluate second argument.
opList$(0) = "set"
opList$(2) = eForm$(opList$(2))
END IF
svarMem$ = selPairE$(varMem$, 1)
svarMem1$ = svarMem$ 'preserve.
DIM savMem$(lenPair(svarMem$) - 1)
memMark = 0
finSrch = 0
DO
'search for symbol to check.
tvarMem$ = selPairE$(svarMem$, 1)
IF selPairE$(tvarMem$, 1) = opList$(1) THEN
'delete current value stored, restore any other variables and add new.
tvarMem$ = consPair$(opList$(1), consPair$(opList$(2), OB$ + selPairE$(tvarMem$, -2) + CB$))
svarMem$ = consPair$(tvarMem$, OB$ + selPairE$(svarMem$, -1) + CB$)
'can float changed value to most recent by restoring any other variables
'first.
IF memMark > 0 THEN
'2/9/06: ensures something was found before adding it.
FOR inc = 0 TO memMark
'recreate the rest of the variable memory. recheck!
svarMem$ = consPair$(savMem$(inc), svarMem$)
NEXT
END IF
varMem$ = consPair$(svarMem$, OB$ + selPairE$(varMem$, -1) + CB$)
finSrch = 1
ELSE
'symbol not found, discard current car and continue.
svarMem$ = OB$ + selPairE$(svarMem$, -1) + CB$
savMem$(memMark) = tvarMem$
memMark = memMark + 1
IF svarMem$ = EL$ THEN
'variable not found at all, create it. ok.
varMem$ = consPair$(consPair$(consPair$(opList$(1), consPair$(opList$(2), dvarMem$)), selPairE$(varMem$, 1)), OB$ + selPairE$(varMem$, -1) + CB$)
finSrch = 1
END IF
END IF
LOOP WHILE finSrch = 0
applyOp$ = opList$(2)
CASE "if"
'conditional execution. ok.
tForm$ = eForm$(opList$(1))
IF tForm$ <> LF$ THEN
'test evaluates to true, evaluate consequence.
applyOp$ = eForm$(opList$(2))
ELSE
'test is false.
IF eleInd = 3 THEN
'alternative present.
applyOp$ = eForm$(opList$(3))
ELSE
'no alternative.
applyOp$ = LF$
END IF
END IF
CASE "get"
'form gets a byte of data from a given device.
SELECT CASE opList$(1)
'action based on device.
CASE "kbrd"
'return character from keyboard.
DO
'wait for character.
Char$ = INKEY$
LOOP WHILE Char$ = ""
applyOp$ = RIGHT$(Char$, 1)
EXIT FUNCTION
CASE "memory"
'return character from memory.
DEF SEG = VAL(opList$(2))
applyOp$ = CHR$(PEEK(VAL(opList$(3))))
DEF SEG
EXIT FUNCTION
CASE "screen"
'return character or pixel from screen.
END SELECT
CASE "number?"
'number predicate. ok.
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF getDatype$(opList$(1)) = numT$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "symbol?"
'symbol predicate. ok.
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF getDatype$(opList$(1)) = atmT$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "boolean?"
'boolean predicate. ok?
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF getDatype$(opList$(1)) = blnT$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "pair?"
'pair predicate. ok?
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF opList$(1) = EL$ THEN
'empty list isn't a pair.
applyOp$ = LF$
EXIT FUNCTION
END IF
rType$ = getDatype$(opList$(1))
IF rType$ = AP$ OR rType$ = LP$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "null?"
'empty list predicate. ok.
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF opList$(1) = EL$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "procedure?"
'procedure predicate. fix.
IF eleInd < 1 THEN
'no args.
applyOp$ = LF$
EXIT FUNCTION
END IF
IF getDatype$(opList$(1)) = procT$ THEN
applyOp$ = LT$
ELSE
applyOp$ = LF$
END IF
CASE "="
'predicate for numerical equality. ok.
IF getDatype$(opList$(1)) = numT$ AND getDatype$(opList$(2)) = numT$ THEN
'both args are numbers.
IF VAL(opList$(1)) = VAL(opList$(2)) THEN
'both equal.
applyOp$ = LT$
ELSE
'not equal.
applyOp$ = LF$
END IF
ELSE
'non-number found.
void$ = errSig$("num")
applyOp$ = LF$
END IF
CASE "eq?"
'predicate for symbol equality.
void$ = errSig$("!")
applyOp$ = LF$
'under construction.
CASE "eqv?"
'predicate for equality in all objects but lists.
void$ = errSig$("!")
applyOp$ = LF$
'under construction.
CASE "equal?"
'predicate for equality in all objects. ok?
IF opList$(1) = opList$(2) THEN
'both equal.
applyOp$ = LT$
ELSE
'not equal.
applyOp$ = LF$
END IF
CASE "lambda"
'defines a procedure.
applyOp$ = list$
CASE ELSE
'attempts to apply a lambda procedure. ok?
parList$ = selPairE$(opList$(0), 2)
procBody$ = selPairE$(opList$(0), 3)
ltChars$ = " ()."
argRd = 1
DO
'get parameter.
IF parList$ = EL$ THEN
'no parameters.
EXIT DO
END IF
parCar$ = selPairE$(parList$, HEAD)
lenPar = LEN(parCar$)
nPos = 1
DO
'replace parameter with argument.
lenBody = LEN(procBody$)
parPos = INSTR(nPos, procBody$, parCar$)
IF parPos = 0 THEN
'not found.
EXIT DO
END IF
lChar$ = MID$(procBody$, parPos - 1, 1)
tChar$ = MID$(procBody$, parPos + lenPar, 1)
IF INSTR(ltChars$, lChar$) > 0 AND INSTR(ltChars$, tChar$) > 0 THEN
'parameter found.
p1Body$ = xMid$(procBody$, 1, parPos - 1)
p2Body$ = xMid$(procBody$, parPos + lenPar, lenBody)
procBody$ = p1Body$ + opList$(argRd) + p2Body$
ELSE
'not a parameter.
nPos = parPos + 1
END IF
LOOP
parList$ = OB$ + selPairE$(parList$, TAIL) + CB$
argRd = argRd + 1
LOOP
applyOp$ = eForm$(procBody$)
END SELECT
END FUNCTION
FUNCTION consPair$ (fst$, rest$)
'constructs a pair. ok.
' Created:
' 26/8/06
' Last Modified:
' 9/9/06
'
' Input:
' element to be paired.
' rest to be paired to.
' Output:
' constructed pair.
'
' Notes:
' 27/8/06: to prevent the insersion of a space when consing to an empty list.
'
' History:
' 27/8/06: corrected empty list process bug.
' 9/9/06: fixed atomic pair constructor to take other atomic pairs along
' with the atom.
' fixed null car to return cdr unchanged.
'
IF fst$ = "" THEN
'nothing to add.
consPair$ = rest$
EXIT FUNCTION
END IF
rType$ = getDatype$(rest$)
IF rType$ <> LP$ THEN
'construct atomic pair.
consPair$ = fst$ + "." + rest$
ELSE
'construct list pair.
IF rest$ = EL$ THEN
'consing an empty list.
consPair$ = OB$ + fst$ + CB$
ELSE
consPair$ = OB$ + fst$ + " " + MID$(rest$, 2)
END IF
END IF
END FUNCTION
FUNCTION eForm$ (form$)
'evaluates the given form.
'
' Created:
' 16/8/06
' Last Modified:
' 16/8/06
'
' Input:
' expression to be evaluated.
' Output:
' evaluation of the expression.
'
' Notes:
' 20/8/06: modify to check for the special characters ( ) [ ] { } ; , " ' `
' # \ in symbols.
' 26/8/06: modify to use data type check procedure.
' modify to process quoted forms separately (the data type procedure needs
' to be generalized to work with more routines).
'
' History:
'
IF MID$(form$, 1, 1) = "'" THEN
'evaluate quoted expression. ok.
eForm$ = MID$(form$, 2)
END IF
SELECT CASE MID$(form$, 1, 1)
'evaluates the form based on its type.
CASE OB$
'evaluates a list pair.
eForm$ = evPair$(form$)
CASE CHR$(39)
'evaluates a quoted expression. 'ok.
eForm$ = MID$(form$, 2)
'prints expression w/out quote.
CASE "#"
'evaluate a special data structure.
'under development.
STOP
'currently returns an error of "unimplemented future data type".
CASE ELSE
'evaluates an atom. 'ok.
eForm$ = evAtom$(form$)
'passes control to atom evaluation.
END SELECT
END FUNCTION
FUNCTION errSig$ (eCode$)
'signals errors.
' Created:
' 26/8/06
' Last Modified:
' 1/9/06
'
' Input:
' error code.
' Output:
' nothing.
'
' Notes:
' 1/9/06: modify to handle all user trappable errors and provide recovery
' for those possible by maybe using qbasic listing and methods.
' 7/9/06: modify to take relevant info and create more comprehensive error
' signals and handlers.
'
' History:
' 7/9/06: added some messages.
'
STOP
SELECT CASE eCode$
'create message based on code.
CASE "var"
'unbound.
errt$ = "unbound variable."
CASE "proc"
'procedure not found.
errt$ = "undefined procedure."
CASE "dbz"
'division.
errt$ = "division by zero!"
CASE "lst"
'list.
errt$ = "not of type list."
CASE ELSE
'unknown error.
errt$ = "unknown or fatal error!"