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Beau­tiful Racket / tuto­rials

Go with the flow: basic

But does it work? Let’s start with the all-time BASIC favorite:

#lang basic
10 print "HELLO WORLD!"
20 goto 10
℘
10 print "HELLO WORLD!"
20 goto 10
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HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
···
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
HELLO WORLD!
···
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Let’s also try our orig­inal sample program, which was designed to cause extra grief for the tokenizer and the parser:

#lang basic
30 rem print 'ignored'
35
50 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5 rem ignored
10 print "o" ; "n" ; "e"
20 print : goto 60.0 : end
#lang basic
30 rem print 'ignored'
35
50 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5 rem ignored
10 print "o" ; "n" ; "e"
20 print : goto 60.0 : end
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one

three
4
one

three
4
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Let’s also see how our source loca­tions worked out. Suppose we change line number 50 to be 30:

#lang basic
30 rem print 'ignored'
35
30 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5
10 print "o" ; "n" ; "e"
20 print : goto 60.0 : end
#lang basic
30 rem print 'ignored'
35
30 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5
10 print "o" ; "n" ; "e"
20 print : goto 60.0 : end
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DrRacket will raise the error we’d expect it to when it finds conflicting defi­n­i­tions, and the offending line is high­lighted correctly:

Let’s switch it back, and see what happens if we intro­duce a bogus line number in the last goto:

#lang basic
30 rem print 'ignored'
35
50 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5
10 print "o" ; "n" ; "e"
20 print : goto 6000.0 : end
#lang basic
30 rem print 'ignored'
35
50 print "never gets here"
40 end
60 print 'three' : print 1.0 + 3
70 goto 11. + 18.5 + .5
10 print "o" ; "n" ; "e"
20 print : goto 6000.0 : end
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error in line 20: line 6000 not found
error in line 20: line 6000 not found
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A side effect of making our lines into func­tions is that they’re avail­able in the DrRacket REPL like any other func­tion. For instance:

(line-10)
one
(line-60)
three
4
(line-70)
uncaught exception: #<change-line-signal>
> (line-10)
one
> (line-60)
three
4
> (line-70)
uncaught excep­tion: #<change-line-signal>
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This last result makes sense: we’re trying to execute a goto outside the context of the program loop.

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