segments->painter fails with list from #lang sicp

Question

segments->painter fails with list from #lang sicp

vdloo opened this issue 10 years ago · 10 comments

Hi,

I ran into a problem while doing exercise 2.49 from SICP. The segments->painter procedure from prmpnt.scm gives a contract violation if it's given a list defined in the sicp lang.

#lang sicp
(#%require sicp-pict)

(define (make-vect x y)
  (cons x y))

(define (xcor-vect vect)
  (car vect))

(define (ycor-vect vect)
  (cdr vect))

(define (make-segment vect-origin-to-start vect-origin-to-end)
  (cons vect-origin-to-start vect-origin-to-end))

(define outline-segments
  (segments->painter
   (list ; <- culprit
    (make-segment (make-vect 0.0 0.0) (make-vect 0.0 0.99))
    (make-segment (make-vect 0.0 0.0) (make-vect 0.99 0.0))
    (make-segment (make-vect 0.99 0.0) (make-vect 0.99 0.99))
    (make-segment (make-vect 0.0 0.99) (make-vect 0.99 0.99)))))

(paint outline-segments)


for-each: contract violation
  expected: list?
  given: (mcons (mcons '(0.0 . 0.0) '(0.0 . 0.99)) (mcons (mcons '(0.0 . 0.0) '(0.99 . 0.0)) (mcons (mcons '(0.99 . 0.0) '(0.99 . 0.99)) (mcons (mcons '(0.0 . 0.99) '(0.99 . 0.99)) '()))))
  argument position: 2nd

The violation occurs here. The snippet above works if instead of using #lang sicp, #lang racket/base is used.

Answer 1 · 2016-03-26T13:30:04.000Z

Hi Rick,

This is a bit subtle. The language #lang sicp uses mutable pairs while
#lang racket uses immutable pairs.
If you create a list in #lang sicp you will get a list consisting of
mutable pairs (printed as mcons) and
since the segments->painter is expecting a Racket list consisting of
immutable pairs (printed as cons) you get
the error.

Best advice: Use #lang racket to work with the painter language.

/Jens Axel

2016-03-26 12:55 GMT+01:00 Rick van de Loo notifications@github.com:

Hi,

I ran into a problem while doing exercise 2.49 from SICP. The
segments->painter procedure from prmpnt.scm gives a contract violation if
it's given a list defined in the sicp lang.

#lang sicp
(#%require sicp-pict)

(define (make-vect x y)
(cons x y))

(define (xcor-vect vect)
(car vect))

(define (ycor-vect vect)
(cdr vect))

(define (make-segment vect-origin-to-start vect-origin-to-end)
(cons vect-origin-to-start vect-origin-to-end))

(define outline-segments
(segments->painter
(list ; <- culprit
(make-segment (make-vect 0.0 0.0) (make-vect 0.0 0.99))
(make-segment (make-vect 0.0 0.0) (make-vect 0.99 0.0))
(make-segment (make-vect 0.99 0.0) (make-vect 0.99 0.99))
(make-segment (make-vect 0.0 0.99) (make-vect 0.99 0.99)))))

(paint outline-segments)

for-each: contract violation
expected: list?
given: (mcons (mcons '(0.0 . 0.0) '(0.0 . 0.99)) (mcons (mcons '(0.0 . 0.0) '(0.99 . 0.0)) (mcons (mcons '(0.99 . 0.0) '(0.99 . 0.99)) (mcons (mcons '(0.0 . 0.99) '(0.99 . 0.99)) '()))))
argument position: 2nd

The violation occurs here
https://github.com/sicp-lang/sicp/blob/master/sicp-pict/prmpnt.scm#L114.
The snippet above works if instead of using #lang sicp, #lang racket/base
is used.

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#4

Jens Axel Søgaard

Answer 2 · 2016-03-26T13:36:07.000Z

I'll see if I can get segments->painter to accept both kind of lists.

/Jens Axel

2016-03-26 14:29 GMT+01:00 Jens Axel Søgaard jensaxel@soegaard.net:

Hi Rick,

This is a bit subtle. The language #lang sicp uses mutable pairs while
#lang racket uses immutable pairs.
If you create a list in #lang sicp you will get a list consisting of
mutable pairs (printed as mcons) and
since the segments->painter is expecting a Racket list consisting of
immutable pairs (printed as cons) you get
the error.

Best advice: Use #lang racket to work with the painter language.

/Jens Axel

2016-03-26 12:55 GMT+01:00 Rick van de Loo notifications@github.com:

Hi,

I ran into a problem while doing exercise 2.49 from SICP. The
segments->painter procedure from prmpnt.scm gives a contract violation if
it's given a list defined in the sicp lang.

#lang sicp
(#%require sicp-pict)

(define (make-vect x y)
(cons x y))

(define (xcor-vect vect)
(car vect))

(define (ycor-vect vect)
(cdr vect))

(define (make-segment vect-origin-to-start vect-origin-to-end)
(cons vect-origin-to-start vect-origin-to-end))

(define outline-segments
(segments->painter
(list ; <- culprit
(make-segment (make-vect 0.0 0.0) (make-vect 0.0 0.99))
(make-segment (make-vect 0.0 0.0) (make-vect 0.99 0.0))
(make-segment (make-vect 0.99 0.0) (make-vect 0.99 0.99))
(make-segment (make-vect 0.0 0.99) (make-vect 0.99 0.99)))))

(paint outline-segments)

for-each: contract violation
expected: list?
given: (mcons (mcons '(0.0 . 0.0) '(0.0 . 0.99)) (mcons (mcons '(0.0 . 0.0) '(0.99 . 0.0)) (mcons (mcons '(0.99 . 0.0) '(0.99 . 0.99)) (mcons (mcons '(0.0 . 0.99) '(0.99 . 0.99)) '()))))
argument position: 2nd

The violation occurs here
https://github.com/sicp-lang/sicp/blob/master/sicp-pict/prmpnt.scm#L114.
The snippet above works if instead of using #lang sicp, #lang racket/base
is used.

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#4

Jens Axel Søgaard

Jens Axel Søgaard

Answer 3 · 2016-03-26T13:48:32.000Z

This commit
0da6a81

fixes the problem.

Update the sicp package either using raco or the DrRacket package manager
to get the changes.

Thanks for the bug report.

/Jens Axel

2016-03-26 14:36 GMT+01:00 Jens Axel Søgaard jensaxel@soegaard.net:

I'll see if I can get segments->painter to accept both kind of lists.

/Jens Axel

2016-03-26 14:29 GMT+01:00 Jens Axel Søgaard jensaxel@soegaard.net:

Hi Rick,

This is a bit subtle. The language #lang sicp uses mutable pairs while
#lang racket uses immutable pairs.
If you create a list in #lang sicp you will get a list consisting of
mutable pairs (printed as mcons) and
since the segments->painter is expecting a Racket list consisting of
immutable pairs (printed as cons) you get
the error.

Best advice: Use #lang racket to work with the painter language.

/Jens Axel

2016-03-26 12:55 GMT+01:00 Rick van de Loo notifications@github.com:

Hi,

I ran into a problem while doing exercise 2.49 from SICP. The
segments->painter procedure from prmpnt.scm gives a contract violation if
it's given a list defined in the sicp lang.

#lang sicp
(#%require sicp-pict)

(define (make-vect x y)
(cons x y))

(define (xcor-vect vect)
(car vect))

(define (ycor-vect vect)
(cdr vect))

(define (make-segment vect-origin-to-start vect-origin-to-end)
(cons vect-origin-to-start vect-origin-to-end))

(define outline-segments
(segments->painter
(list ; <- culprit
(make-segment (make-vect 0.0 0.0) (make-vect 0.0 0.99))
(make-segment (make-vect 0.0 0.0) (make-vect 0.99 0.0))
(make-segment (make-vect 0.99 0.0) (make-vect 0.99 0.99))
(make-segment (make-vect 0.0 0.99) (make-vect 0.99 0.99)))))

(paint outline-segments)

for-each: contract violation
expected: list?
given: (mcons (mcons '(0.0 . 0.0) '(0.0 . 0.99)) (mcons (mcons '(0.0 . 0.0) '(0.99 . 0.0)) (mcons (mcons '(0.99 . 0.0) '(0.99 . 0.99)) (mcons (mcons '(0.0 . 0.99) '(0.99 . 0.99)) '()))))
argument position: 2nd

The violation occurs here
https://github.com/sicp-lang/sicp/blob/master/sicp-pict/prmpnt.scm#L114.
The snippet above works if instead of using #lang sicp, #lang
racket/base is used.

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#4

Jens Axel Søgaard

Jens Axel Søgaard

Jens Axel Søgaard

Answer 4 · 2016-03-26T16:34:35.000Z

Great! Thanks for the quick reply :)

Answer 5 · 2020-03-05T22:37:37.000Z

GOOD GRIEF! I spent a whole day wondering what in the world was wrong with my code.

Thank you for the question and the answer. I wish I had checked sooner.

I can concur that declaring #lang racket instead of #lang sicp is the fix.

I pushed a commit to my repo, fixing the language declaration, and with details in the commit message.

Use 'racket', not 'sicp' in ex2-44 picture lang file adityaathalye/sicp@c8b62c3

Answer 6 · 2020-03-06T00:44:14.000Z

Hmm. Provided that @soegaard made segments->painter to accept both mutable and immutable lists, there should not be any problem, right? Can you provide us a code example that leads to a problem?

Answer 7 · 2020-03-06T02:02:59.000Z

Hmm. Provided that @soegaard made segments->painter to accept both mutable and immutable lists, there should not be any problem, right? Can you provide us a code example that leads to a problem?

Updated my earlier comment to keep things in context: #4 (comment)

Answer 8 · 2020-03-06T02:12:56.000Z

That's a regression caused by me (see ce24736). I will push a fix soon, and you should be able to switch back to #lang sicp after that (feel free to keep using #lang racket, too!)

Answer 9 · 2020-04-19T20:02:59.000Z

I believe this is fixed by 4152068. Closing. Feel free to reopen it if you find any other issues.

Answer 10 · 2020-04-20T01:56:28.000Z

I just updated lang-sicp locally and can confirm SHA 4152068 fixes the bug. Thank you!