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Dylan Design Notes: #16: List Issues (Change)

Dylan Design Notes

#16: List Issues (Change)

Version 1, March 1993

In this design note, Dylan's specification of the <list> type is made more consistent with the rest of the language, mysterious abbreviations and redundant list-only operations are removed, and handling of improper lists is clarified. These changes make the language more accessible to our primary audience of programmers who have not used dynamic languages before.


Make the following changes to the Dylan manual, pages 115-117:

Rename cons to pair. Rename car and cdr to head and tail respectively, and rename the corresponding setters to head-setter and tail-setter. Remove the compound functions caar through cdddr and the corresponding setter operations.

Remove list*. Use (pair a (pair b c)) instead of (list* a b c).

Remove find-pair.

Remove append. Use concatenate instead. Replace the definition of concatenate on page 108 with the following:

concatenate sequence1 #rest sequences=> new-sequence        [Generic Function]
concatenate returns a new sequence containing all the elements of all the sequences, in order. new-sequence may non-destructively share structure with any of the input sequences, but it is not guaranteed to do so.

Specify that member? on lists always returns a boolean. Remove the definition of member? on page 117, and replace the definition of member? on page 103 with the following:

member? value collection #key test => boolean [Generic Function] member? returns #t if and only if collection contains value (as determined by test, which defaults to id?). The test function may be non-commutative: it is always called with value as its first argument and an element from collection as its second argument.

? (define flavors '(pistachio chocolate pumpkin))
flavors
? (member? 'chocolate flavors)
#t
? (member? 'banana flavors)
#f
Add the following description of the class <empty-list>:
<empty-list>        [Sealed Instantiable Class]
The class <empty-list> has only one instance, the empty list. The empty list is a direct instance of <empty-list> and an indirect instance of <list>. Note that <empty-list> is not id? to (singleton '()).
? (object-class '())
<empty-list>
? (define-method f (x <empty-list>) 1)
? (define-method f (x (singleton '())) 2)
? (define-method f (x <list>) 3)
? (f '())
2
? (f '(chocolate vanilla))
3
An improper list is a list that is not terminated by the empty list, either because it is terminated by something that is not a list, or because it is circular and thus non-terminating. Except when their behavior on improper lists is documented explicitly, collection or sequence functions are not guaranteed to return an answer when an improper list is used as a collection or a sequence. At the implementation's option, these functions may return the correct result, signal a <type-error>, or (in the case of a circular list) fail to return.
? (size '(1  2 . 3)) 
   ; signals a <type-error> (there is no correct result)
? (size (bind ((x (pair 1 1))) (set! (tail x) x)))
#f
? (member? 1 (bind ((x (pair 1 1))) (set! (tail x) x)))
#t ; may also signal a <type-error> or never return
? (member? 0 (bind ((x (pair 1 1))) (set! (tail x) x)))
#f ; may also signal a <type-error> or never return
? (element '(1 2 . 3) 1)
2  ; may also signal a <type-error> 
? (element '(1 2 . 3) 2)
   ; signals a key out of range error or a <type-error>
? (element '(1 2 . 3) 2 default: 0)
0  ; may also signal a <type-error>

Notes

The mysterious abbreviations car and cdr have no meaning to most people. The name cons does not have an apparent meaning either, and it is not obviously related to the class, <pair>, that it instantiates.

The compound functions caar through cdddr seem like holdovers from the days before object-oriented programming, when data structures were built up from pairs, using these functions as accessors, and not encapsulated in abstractions.

list* is an obscure function. It does the same thing as cons when called with two arguments, so one of them is redundant.

append does almost the same thing as concatenate, making it seem redundant. The result of append is specified to share structure with the last argument; the book does not specify whether the result of concatenate must, might, or must not share structure with an argument. We feel that the decision whether to share structure in concatenate is regarded as an implementation optimization decision, rather than language semantics. This assumes that users want structure-sharing only to optimize space, not because they rely on modifying the shared structure.

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