car implementation in scheme

I am trying to write by myself the cons function in scheme. I have written this code:

(define (car. z)

    (z (lambda (p q) p)))

and I am trying to run :

(car. '(1 2 3))

I expect to get the number 1, but it does not work properly.

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

1

In your definition of car. the argument z is used as a function applied to another function. In the call you pass instead a list, not a function.

– Renzo
Jan 1 at 10:23

5

car. wants a "lambda-encoded" list, not a Scheme list. You need to implement the corresponding cons. operation, so you can create such lists.

– molbdnilo
Jan 1 at 10:25

1

As @molbdnilo said in the comment, you must decide if you want to represent lists as functional objects (the definition of the function car.), or as data structures (as in your call), and act uniformely. If you represent lists as functional objects, you must define first cons, and pass the result of this function to car..

– Renzo
Jan 1 at 10:28

1

In practice the genuine car, cdr and cons are builtins (and they have to be). Read Queinnec's Lisp In Small Pieces

– Basile Starynkevitch
Jan 1 at 10:42

2

@user10853826, you can find everything in this great free book online. Or see for instance this SO question

– Renzo
Jan 1 at 11:28

|
show 7 more comments

I am trying to write by myself the cons function in scheme. I have written this code:

(define (car. z)

    (z (lambda (p q) p)))

and I am trying to run :

(car. '(1 2 3))

I expect to get the number 1, but it does not work properly.

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

1

In your definition of car. the argument z is used as a function applied to another function. In the call you pass instead a list, not a function.

– Renzo
Jan 1 at 10:23

5

car. wants a "lambda-encoded" list, not a Scheme list. You need to implement the corresponding cons. operation, so you can create such lists.

– molbdnilo
Jan 1 at 10:25

1

As @molbdnilo said in the comment, you must decide if you want to represent lists as functional objects (the definition of the function car.), or as data structures (as in your call), and act uniformely. If you represent lists as functional objects, you must define first cons, and pass the result of this function to car..

– Renzo
Jan 1 at 10:28

1

In practice the genuine car, cdr and cons are builtins (and they have to be). Read Queinnec's Lisp In Small Pieces

– Basile Starynkevitch
Jan 1 at 10:42

2

@user10853826, you can find everything in this great free book online. Or see for instance this SO question

– Renzo
Jan 1 at 11:28

|
show 7 more comments

I am trying to write by myself the cons function in scheme. I have written this code:

(define (car. z)

    (z (lambda (p q) p)))

and I am trying to run :

(car. '(1 2 3))

I expect to get the number 1, but it does not work properly.

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

I am trying to write by myself the cons function in scheme. I have written this code:

(define (car. z)

    (z (lambda (p q) p)))

and I am trying to run :

(car. '(1 2 3))

I expect to get the number 1, but it does not work properly.

scheme racket

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

edited Jan 1 at 10:35

rsm

1,26431522

edited Jan 1 at 10:35

rsm

1,26431522

edited Jan 1 at 10:35

rsm

1,26431522

asked Jan 1 at 10:07

user10853826

asked Jan 1 at 10:07

user10853826

asked Jan 1 at 10:07

user10853826

1

In your definition of car. the argument z is used as a function applied to another function. In the call you pass instead a list, not a function.

– Renzo
Jan 1 at 10:23

5

car. wants a "lambda-encoded" list, not a Scheme list. You need to implement the corresponding cons. operation, so you can create such lists.

– molbdnilo
Jan 1 at 10:25

1

As @molbdnilo said in the comment, you must decide if you want to represent lists as functional objects (the definition of the function car.), or as data structures (as in your call), and act uniformely. If you represent lists as functional objects, you must define first cons, and pass the result of this function to car..

– Renzo
Jan 1 at 10:28

1

In practice the genuine car, cdr and cons are builtins (and they have to be). Read Queinnec's Lisp In Small Pieces

– Basile Starynkevitch
Jan 1 at 10:42

2

@user10853826, you can find everything in this great free book online. Or see for instance this SO question

– Renzo
Jan 1 at 11:28

|
show 7 more comments

1

In your definition of car. the argument z is used as a function applied to another function. In the call you pass instead a list, not a function.

– Renzo
Jan 1 at 10:23

5

car. wants a "lambda-encoded" list, not a Scheme list. You need to implement the corresponding cons. operation, so you can create such lists.

– molbdnilo
Jan 1 at 10:25

1

As @molbdnilo said in the comment, you must decide if you want to represent lists as functional objects (the definition of the function car.), or as data structures (as in your call), and act uniformely. If you represent lists as functional objects, you must define first cons, and pass the result of this function to car..

– Renzo
Jan 1 at 10:28

1

In practice the genuine car, cdr and cons are builtins (and they have to be). Read Queinnec's Lisp In Small Pieces

– Basile Starynkevitch
Jan 1 at 10:42

2

@user10853826, you can find everything in this great free book online. Or see for instance this SO question

– Renzo
Jan 1 at 11:28

In your definition of car. the argument z is used as a function applied to another function. In the call you pass instead a list, not a function.

– Renzo
Jan 1 at 10:23

car. wants a "lambda-encoded" list, not a Scheme list. You need to implement the corresponding cons. operation, so you can create such lists.

– molbdnilo
Jan 1 at 10:25

As @molbdnilo said in the comment, you must decide if you want to represent lists as functional objects (the definition of the function car.), or as data structures (as in your call), and act uniformely. If you represent lists as functional objects, you must define first cons, and pass the result of this function to car..

– Renzo
Jan 1 at 10:28

In practice the genuine car, cdr and cons are builtins (and they have to be). Read Queinnec's Lisp In Small Pieces

– Basile Starynkevitch
Jan 1 at 10:42

@user10853826, you can find everything in this great free book online. Or see for instance this SO question

– Renzo
Jan 1 at 11:28

|
show 7 more comments

2 Answers
2

active

oldest

votes

When you implement language data structures you need to supply constructors and accessors that conform to the contract:

(car (cons 1 2))   ; ==> 1

(cdr (cons 1 2))   ; ==> 2

(pair? (cons 1 2)) ; ==> 2

Here is an example:

(define (cons a d)

  (vector a d))

(define (car p)

  (vector-ref p 0))

(define (cdr p)

  (vector-ref p 1))

Now if you make an implementation you would implement read to be conformant to this way of doing pairs so that '(1 2 3) would create the correct data structure the simple rules above is still the same.

From looking at car I imagine cons looks like this:

(define (cons a d)

  (lambda (p) (p a d)))

It works with closures. Now A stack machine implementation of Scheme would analyze the code for free variables living passed their scope and thus create them as boxes. Closures containing a, and d aren't much different than vectors.

I urge you to implement a minimalistic Scheme interpreter. First in Scheme since you can use the host language, then a different than a lisp language. You can even do it in an esoteric language, but it is very time consuming.

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

add a comment |

Sylwester's answer is great. Here's another possible implementation of null, null?, cons, car, cdr -

(define null 'null)



(define (null? xs)

  (eq? null xs))



(define (cons a b)

  (define (dispatch message)

    (match message

      ('car a)

      ('cdr b)

      (_ (error 'cons "unsupported message" message))

  dispatch)



(define (car xs)

  (if (null? xs)

      (error 'car "cannot call car on an empty pair")

      (xs 'car)))



(define (cdr xs)

  (if (null? xs)

      (error 'cdr "cannot call cdr on an empty pair")

      (xs 'cdr)))

It works like this -

(define xs (cons 'a (cons 'b (cons 'c null))))



(printf "~a -> ~a -> ~an"

        (car xs)

        (car (cdr xs))

        (car (cdr (cdr xs))))

;; a -> b -> c

It raises errors in these scenarios -

(cdr null)

; car: cannot call car on an empty pair



(cdr null)

; cdr: cannot call cdr on an empty pair



((cons 'a 'b) 'foo)

;; cons: unsupported dispatch: foo

define/match adds a little sugar, if you like sweet things -

(define (cons a b)

  (define/match (dispatch msg)

    (('car) a)

    (('cdr) b)

    (('pair?) #t)

    ((_) (error 'cons "unsupported dispatch: ~a" msg)))

  dispatch)



((cons 1 2) 'car)   ;; 1

((cons 1 2) 'cdr)   ;; 2

((cons 1 2) 'pair?) ;; #t

((cons 1 2) 'foo)   ;; cons: unsupported dispatch: foo

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

When you implement language data structures you need to supply constructors and accessors that conform to the contract:

(car (cons 1 2))   ; ==> 1

(cdr (cons 1 2))   ; ==> 2

(pair? (cons 1 2)) ; ==> 2

Here is an example:

(define (cons a d)

  (vector a d))

(define (car p)

  (vector-ref p 0))

(define (cdr p)

  (vector-ref p 1))

From looking at car I imagine cons looks like this:

(define (cons a d)

  (lambda (p) (p a d)))

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

add a comment |

When you implement language data structures you need to supply constructors and accessors that conform to the contract:

(car (cons 1 2))   ; ==> 1

(cdr (cons 1 2))   ; ==> 2

(pair? (cons 1 2)) ; ==> 2

Here is an example:

(define (cons a d)

  (vector a d))

(define (car p)

  (vector-ref p 0))

(define (cdr p)

  (vector-ref p 1))

From looking at car I imagine cons looks like this:

(define (cons a d)

  (lambda (p) (p a d)))

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

add a comment |

When you implement language data structures you need to supply constructors and accessors that conform to the contract:

(car (cons 1 2))   ; ==> 1

(cdr (cons 1 2))   ; ==> 2

(pair? (cons 1 2)) ; ==> 2

Here is an example:

(define (cons a d)

  (vector a d))

(define (car p)

  (vector-ref p 0))

(define (cdr p)

  (vector-ref p 1))

From looking at car I imagine cons looks like this:

(define (cons a d)

  (lambda (p) (p a d)))

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

When you implement language data structures you need to supply constructors and accessors that conform to the contract:

(car (cons 1 2))   ; ==> 1

(cdr (cons 1 2))   ; ==> 2

(pair? (cons 1 2)) ; ==> 2

Here is an example:

(define (cons a d)

  (vector a d))

(define (car p)

  (vector-ref p 0))

(define (cdr p)

  (vector-ref p 1))

From looking at car I imagine cons looks like this:

(define (cons a d)

  (lambda (p) (p a d)))

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

edited Jan 1 at 19:05

answered Jan 1 at 17:43

Sylwester

34.9k22956

answered Jan 1 at 17:43

Sylwester

34.9k22956

answered Jan 1 at 17:43

Sylwester

34.9k22956

add a comment |

Sylwester's answer is great. Here's another possible implementation of null, null?, cons, car, cdr -

(define null 'null)



(define (null? xs)

  (eq? null xs))



(define (cons a b)

  (define (dispatch message)

    (match message

      ('car a)

      ('cdr b)

      (_ (error 'cons "unsupported message" message))

  dispatch)



(define (car xs)

  (if (null? xs)

      (error 'car "cannot call car on an empty pair")

      (xs 'car)))



(define (cdr xs)

  (if (null? xs)

      (error 'cdr "cannot call cdr on an empty pair")

      (xs 'cdr)))

It works like this -

(define xs (cons 'a (cons 'b (cons 'c null))))



(printf "~a -> ~a -> ~an"

        (car xs)

        (car (cdr xs))

        (car (cdr (cdr xs))))

;; a -> b -> c

It raises errors in these scenarios -

(cdr null)

; car: cannot call car on an empty pair



(cdr null)

; cdr: cannot call cdr on an empty pair



((cons 'a 'b) 'foo)

;; cons: unsupported dispatch: foo

define/match adds a little sugar, if you like sweet things -

(define (cons a b)

  (define/match (dispatch msg)

    (('car) a)

    (('cdr) b)

    (('pair?) #t)

    ((_) (error 'cons "unsupported dispatch: ~a" msg)))

  dispatch)



((cons 1 2) 'car)   ;; 1

((cons 1 2) 'cdr)   ;; 2

((cons 1 2) 'pair?) ;; #t

((cons 1 2) 'foo)   ;; cons: unsupported dispatch: foo

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

add a comment |

Sylwester's answer is great. Here's another possible implementation of null, null?, cons, car, cdr -

(define null 'null)



(define (null? xs)

  (eq? null xs))



(define (cons a b)

  (define (dispatch message)

    (match message

      ('car a)

      ('cdr b)

      (_ (error 'cons "unsupported message" message))

  dispatch)



(define (car xs)

  (if (null? xs)

      (error 'car "cannot call car on an empty pair")

      (xs 'car)))



(define (cdr xs)

  (if (null? xs)

      (error 'cdr "cannot call cdr on an empty pair")

      (xs 'cdr)))

It works like this -

(define xs (cons 'a (cons 'b (cons 'c null))))



(printf "~a -> ~a -> ~an"

        (car xs)

        (car (cdr xs))

        (car (cdr (cdr xs))))

;; a -> b -> c

It raises errors in these scenarios -

(cdr null)

; car: cannot call car on an empty pair



(cdr null)

; cdr: cannot call cdr on an empty pair



((cons 'a 'b) 'foo)

;; cons: unsupported dispatch: foo

define/match adds a little sugar, if you like sweet things -

(define (cons a b)

  (define/match (dispatch msg)

    (('car) a)

    (('cdr) b)

    (('pair?) #t)

    ((_) (error 'cons "unsupported dispatch: ~a" msg)))

  dispatch)



((cons 1 2) 'car)   ;; 1

((cons 1 2) 'cdr)   ;; 2

((cons 1 2) 'pair?) ;; #t

((cons 1 2) 'foo)   ;; cons: unsupported dispatch: foo

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

add a comment |

Sylwester's answer is great. Here's another possible implementation of null, null?, cons, car, cdr -

(define null 'null)



(define (null? xs)

  (eq? null xs))



(define (cons a b)

  (define (dispatch message)

    (match message

      ('car a)

      ('cdr b)

      (_ (error 'cons "unsupported message" message))

  dispatch)



(define (car xs)

  (if (null? xs)

      (error 'car "cannot call car on an empty pair")

      (xs 'car)))



(define (cdr xs)

  (if (null? xs)

      (error 'cdr "cannot call cdr on an empty pair")

      (xs 'cdr)))

It works like this -

(define xs (cons 'a (cons 'b (cons 'c null))))



(printf "~a -> ~a -> ~an"

        (car xs)

        (car (cdr xs))

        (car (cdr (cdr xs))))

;; a -> b -> c

It raises errors in these scenarios -

(cdr null)

; car: cannot call car on an empty pair



(cdr null)

; cdr: cannot call cdr on an empty pair



((cons 'a 'b) 'foo)

;; cons: unsupported dispatch: foo

define/match adds a little sugar, if you like sweet things -

(define (cons a b)

  (define/match (dispatch msg)

    (('car) a)

    (('cdr) b)

    (('pair?) #t)

    ((_) (error 'cons "unsupported dispatch: ~a" msg)))

  dispatch)



((cons 1 2) 'car)   ;; 1

((cons 1 2) 'cdr)   ;; 2

((cons 1 2) 'pair?) ;; #t

((cons 1 2) 'foo)   ;; cons: unsupported dispatch: foo

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

Sylwester's answer is great. Here's another possible implementation of null, null?, cons, car, cdr -

(define null 'null)



(define (null? xs)

  (eq? null xs))



(define (cons a b)

  (define (dispatch message)

    (match message

      ('car a)

      ('cdr b)

      (_ (error 'cons "unsupported message" message))

  dispatch)



(define (car xs)

  (if (null? xs)

      (error 'car "cannot call car on an empty pair")

      (xs 'car)))



(define (cdr xs)

  (if (null? xs)

      (error 'cdr "cannot call cdr on an empty pair")

      (xs 'cdr)))

It works like this -

(define xs (cons 'a (cons 'b (cons 'c null))))



(printf "~a -> ~a -> ~an"

        (car xs)

        (car (cdr xs))

        (car (cdr (cdr xs))))

;; a -> b -> c

It raises errors in these scenarios -

(cdr null)

; car: cannot call car on an empty pair



(cdr null)

; cdr: cannot call cdr on an empty pair



((cons 'a 'b) 'foo)

;; cons: unsupported dispatch: foo

define/match adds a little sugar, if you like sweet things -

(define (cons a b)

  (define/match (dispatch msg)

    (('car) a)

    (('cdr) b)

    (('pair?) #t)

    ((_) (error 'cons "unsupported dispatch: ~a" msg)))

  dispatch)



((cons 1 2) 'car)   ;; 1

((cons 1 2) 'cdr)   ;; 2

((cons 1 2) 'pair?) ;; #t

((cons 1 2) 'foo)   ;; cons: unsupported dispatch: foo

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

edited Jan 2 at 21:12

answered Jan 2 at 21:00

user633183

70.8k21140180

answered Jan 2 at 21:00

user633183

70.8k21140180

answered Jan 2 at 21:00

user633183

70.8k21140180

add a comment |

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