This was done just for fun!
CLEMENY is a (very basic) BASIC to Common Lisp compiler. It supports REM, LET, IF .. THEN, GOTO, PRINT, and END, as well as simple arithmetics. All lines must be numbered, and may appear out of order.
The name CLEMENY is named after John G. Kemeny, the pioneer who co-invented BASIC with Thomas E. Kurtz in 1964.
CLEMENY uses the esrap library to build a grammar for BASIC.
Let's have a look at a sample program to sum all multiples of 3 and 5 below 1000:
0000 REM Count multiples of 3 and 5 below 1000
0010 LET I = 3
0011 LET SUM = 0
9000 PRINT SUM
9001 END
0100 LET TEMP = I % 3
0110 IF TEMP = 0 THEN 400
0200 LET TEMP = I % 5
0210 IF TEMP = 0 THEN 400
0300 LET I = I + 1
0310 IF I = 1000 THEN 9000
0320 GOTO 100
0400 LET SUM = SUM + I
0410 GOTO 300
When parsed this produces an abstract syntax tree in Lisp looking like this:
((0 ("REM" " Count multiples of 3 and 5 below 1000"))
(10 ("LET" "I" 3))
(11 ("LET" "SUM" 0))
(100 ("LET" "TEMP" ("MOD" "I" 3)))
(110 ("IF" ("=" "TEMP" 0) 400))
(200 ("LET" "TEMP" ("MOD" "I" 5)))
(210 ("IF" ("=" "TEMP" 0) 400))
(300 ("LET" "I" ("+" "I" 1)))
(310 ("IF" ("=" "I" 1000) 9000))
(320 ("GOTO" 100))
(400 ("LET" "SUM" ("+" "SUM" "I")))
(410 ("GOTO" 300))
(9000 ("PRINT" "SUM"))
(9001 ("END")))
Compiling this tree into executable Common Lisp code is actually not very hard, once you know about the TAGBODY operator. The compiled program will eventually look like this:
(BLOCK NIL
(LET (TEMP I SUM)
(TAGBODY
0
10
(SETF I 3)
11
(SETF SUM 0)
100
(SETF TEMP (MOD I 3))
110
(WHEN (= TEMP 0) (GO 400))
200
(SETF TEMP (MOD I 5))
210
(WHEN (= TEMP 0) (GO 400))
300
(SETF I (+ I 1))
310
(WHEN (= I 1000) (GO 9000))
320
(GO 100)
400
(SETF SUM (+ SUM I))
410
(GO 300)
9000
(FORMAT T "~A~%" SUM)
9001
(RETURN))))
By sending that code to EVAL the sum of 233168 is printed to the terminal.