Re: Scheme dotted pair equivalent in Clojure
A very nice section of SICP[1] describes how cons/car/cdr can be built only with functions. Translated to Clojure it might look like (defn cons [a b] #(condp = % :car a :cdr b)) (defn car [cons-cell] (cons-cell :car)) (defn cdr [cons-cell] (cons-cell :cdr)) (car (cons :a :b)) => :a (cdr (cons 1 2)) => 2 An excellent example of closures and the use of higher order functions. Pairs can also serve as nice example usage for defprotocol and reify: (defprotocol Pair (car [cons-cell]) (cdr [cons-cell])) (defn cons [a b] (reify Pair (car [_] a) (cdr [_] b))) (car (cons :a :b)) => :a (cdr (cons 1 2)) => 2 [1]: http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-14.html#%_sec_2.1.3 On Saturday, June 16, 2012 6:35:14 PM UTC+3, octopusgrabbus wrote: > > I have a need to learn enough scheme to read it and write a few functions. > I came across dotted pair notation. I am trying to grok it in terms of the > only Lisp I know, Clojure. Does dotted pair notation in Scheme compare to > form in Clojure, and if so, how? > > > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
In the previous post, I accidentally deleted a dot when pasting: user=> (clojure.lang.MapEntry :a 1) should have been user=> (clojure.lang.MapEntry. :a 1) -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
A cons is essentially just a struct with two fields. In Clojure, it's sort
of like:
(defrecord Cons [car cdr])
(defn cons [x y] (Cons. x y))
(defn first [x] (:car x))
(defn rest [x] (:cdr x))
The amazing thing is that you can represent any collection of arbitrary
length, just by nesting these structs with two fields (aka "cons cells"),
e.g., (cons 1 (cons 2 (cons 3 (cons 4 empty
But the real magic happens in the printer, where the printer knows to
follow the chain of conses until it hits empty and print the whole thing
out as a list (1 2 3 4).
In Scheme, there's no restriction on what you can pass to cons. So you can
perfectly legally do (cons 1 2). How should this print? By convention, it
prints as (1 . 2). So that's all a dotted pair is.
Why is it used?
Well, the most common use is in map-like structures. A map is comprised of
key-value pairs, so you might as well store these pairs as (cons key value)
rather than (cons key (cons value empty)). The former saves you memory by
using only one cons instead of two. The former prints as a dotted pair,
since it is not a valid list ending in empty.
Clojure uses a similar trick to save memory in key-value pairs for maps.
user=> (first {:a 1, :b 2})
[a 1]
Even though it prints like a list, this is really not a true list. It is
Clojure's equivalent of a dotted pair, a class called MapEntry which
contains a key and a value.
user=> (type (first {:a 1, :b 2}))
clojure.lang.MapEntry
Note that you can build MapEntry structures directly and access their
components, if you want to achieve a similar effect in your own code:
user=> (clojure.lang.MapEntry :a 1)
[:a 1] ;Prints like a list, but this is not a true list.
user=> (key (clojure.lang.MapEntry. :a 1))
:a
user=> (value (clojure.lang.MapEntry. :a 1))
1
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Re: Scheme dotted pair equivalent in Clojure
On Jun 16, 11:37 am, David Nolen wrote: > Not possible in Clojure and a source of hassle in Scheme and Common Lisp. > If you have dotted pairs you can never know if you have a proper list. I'm probably missing some context, but I've heard this argument before and had a hard time understanding it. Clojure is dynamically typed, and you should expect that calling something like 'map with an improper list would blow up just like you would expect this to blow up: user=> (map inc 5) IllegalArgumentException Don't know how to create ISeq from: java.lang.Long clojure.lang.RT.seqFrom (RT.java:494) With dotted pairs I would expect something like: user=> (map inc '(5 6 . 7))) IllegalArgumentException Don't know how to create ISeq from: java.lang.Long clojure.lang.RT.seqFrom (RT.java:494) And in Racket you get: > (map (lambda (x) (+ x 1)) 2) map: expects type as 2nd argument, given: 2; other arguments were: # > (map (lambda (x) (+ x 1)) '(1 . 2)) map: expects type as 2nd argument, given: '(1 . 2); other arguments were: # I'm not necessarily arguing that Clojure should have dotted pairs. I think something like [1 2] works well as a substitute. Plus Clojure already uses . for other things and the overloading might be confusing. I'm just not sure I understand the typing argument for a dynamically typed language. Paul -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
> > I have a need to learn enough scheme to read it and write a few functions. > I came across dotted pair notation. I am trying to grok it in terms of the > only Lisp I know, Clojure. Does dotted pair notation in Scheme compare to > form in Clojure, and if so, how? > As David notes, dotted-pairs are not allowed in Clojure because they allow for improper lists, which would break many of the nice HOFs we like to use (map, filter, etc). Dotted-pairs exist in Scheme because of the structure of the original cons-cell, essentially a structure with two pointers, car and cdr. In a proper list, car will point to some value and cdr will point to either another cons-cell or nil. Now suppose you want to encode a list of properties, i.e. key-value pairs. You could make a proper list where each car points to a pair, which would be another cons-cell since that's all we have to work with. This will work very nicely for small-ish property lists. But how should we encode the properties themselves? We could make a proper list of length two, where the first car points to the key and the second points to the value, but that's kinda wasteful. We're using two whole cons-cells when we really just need one: car points to the key and cdr to the value. And there you have it, the historical justification for dotted-pairs. They were very useful to save memory, perhaps other reasons as well, but this is the main one I know. Of course with the abundance of RAM on modern machines, this is much less of a concern, so Clojure enforces proper lists to increase safety :) Hope this was somewhat understandable, I'm too lazy to try to draw ASCII box-and-pointer diagrams right now.. Eric -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
dotted pairs and cons in Scheme and CL both allow you specify an improper tail. This has some historical utility if you are using cons as a way to build up non-list data structures. Scheme and CL both provide better facilities for data structures than using cons. As far as I know lists are just conses with proper tails. David On Sat, Jun 16, 2012 at 12:03 PM, octopusgrabbus wrote: > Thanks for answering. I am taking from your answer that I can be > Lisp/Clojure-esque in Scheme and not worry about dotted pairs. That is, I > can make a grid out of lists, rather than using cons to construct them. > That's what I'm trying to do. > > > On Saturday, June 16, 2012 11:35:14 AM UTC-4, octopusgrabbus wrote: >> >> I have a need to learn enough scheme to read it and write a few >> functions. I came across dotted pair notation. I am trying to grok it in >> terms of the only Lisp I know, Clojure. Does dotted pair notation in Scheme >> compare to form in Clojure, and if so, how? >> >> >> -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
Thanks for answering. I am taking from your answer that I can be Lisp/Clojure-esque in Scheme and not worry about dotted pairs. That is, I can make a grid out of lists, rather than using cons to construct them. That's what I'm trying to do. On Saturday, June 16, 2012 11:35:14 AM UTC-4, octopusgrabbus wrote: > > I have a need to learn enough scheme to read it and write a few functions. > I came across dotted pair notation. I am trying to grok it in terms of the > only Lisp I know, Clojure. Does dotted pair notation in Scheme compare to > form in Clojure, and if so, how? > > > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Re: Scheme dotted pair equivalent in Clojure
Not possible in Clojure and a source of hassle in Scheme and Common Lisp. If you have dotted pairs you can never know if you have a proper list. David On Sat, Jun 16, 2012 at 11:35 AM, octopusgrabbus wrote: > I have a need to learn enough scheme to read it and write a few functions. > I came across dotted pair notation. I am trying to grok it in terms of the > only Lisp I know, Clojure. Does dotted pair notation in Scheme compare to > form in Clojure, and if so, how? > > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
Scheme dotted pair equivalent in Clojure
I have a need to learn enough scheme to read it and write a few functions. I came across dotted pair notation. I am trying to grok it in terms of the only Lisp I know, Clojure. Does dotted pair notation in Scheme compare to form in Clojure, and if so, how? -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
