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Issue SEQUENCE-TYPE-LENGTH Writeup

Issue:         SEQUENCE-TYPE-LENGTH

References: CLtL p.51, p.249, p.260, p.252, p.354

CONCATENATE, COERCE, MAKE-SEQUENCE, MAP, MERGE

Category: CLARIFICATION

Edit history: 16-Jun-89, version 1, by Moon

Problem description:

In several functions that take a type specifier as an argument and create

a sequence of the specified type, it isn't clear what happens if the type

specifier has an explicit length that doesn't match the length implied by

the other arguments.

Proposal (SEQUENCE-TYPE-LENGTH:MUST-MATCH):

COERCE should signal an error if the new sequence type specifies the

number of elements and the old sequence has a different length.

MAKE-SEQUENCE should signal an error if the sequence type specifies the

number of elements and the size argument is different.

CONCATENATE should signal an error if the sequence type specifies the

number of elements and the sum of the argument lengths is different.

MAP should signal an error if the sequence type specifies the number of

elements and the minimum of the argument lengths is different.

MERGE should signal an error if the sequence type specifies the number of

elements and the sum of the lengths of the two sequence arguments is

different.

Examples:

;; All of the following forms should signal an error

(coerce '(a b c) '(vector * 4))

(coerce #(a b c) '(vector * 4))

(coerce '(a b c) '(vector * 2))

(coerce #(a b c) '(vector * 2))

(coerce "foo" '(string 2))

(coerce #(#\a #\b #\c) '(string 2))

(coerce '(0 1) '(simple-bit-vector 3))

(make-sequence '(vector * 2) 3)

(make-sequence '(vector * 4) 3)

(concatenate '(vector * 2) "a" "bc")

(map '(vector * 4) #'cons "abc" "de")

(merge '(vector * 4) '(1 5) '(2 4 6) #'<)

Rationale:

If CLtL hadn't overlooked this situation, it's likely that it would have

said it "is an error". The best translation of that to ANSI CL error

terminology seemed to be "should signal". There doesn't seem to be any

reason to require signalling this error even in unsafe code. There

doesn't seem to be any reason to define this situation to do something

other than signalling an error, such as ignoring the length in the

type specifier or forcing the sequence to have the correct length by

truncating or extending it with elements of implementation-dependent

value.

Current practice:

Symbolics Genera 7.2 and 7.4 usually ignore the length in the type

specifier in the above situations, but sometimes signal an error.

The type of error signalled is sometimes somewhat random.

Other implementations were not surveyed.

Cost to Implementors:

This does not seem like difficult checking to add. I have not examined

the code in any implementation to try to evaluate what it would cost.

Cost to Users:

None.

Cost of non-adoption:

Aesthetic.

Performance impact:

Probably small, just have to keep track of the length when dealing

with sequence type specifiers in safe code. I have not attempted to

evaluate the exact impact.

Benefits:

Less ambiguity in the language specification. Less deviation among

implementations, hence fewer porting problems.

Esthetics:

Since the length field is present in sequence type specifiers, it

seems unesthetic to ignore it, and even more unesthetic not to say

what is done with it.

Discussion:

Moon doesn't know what error condition is appropriate. TYPE-ERROR

doesn't seem quite appropriate here. One idea is not to say, just let it

be any subtype of ERROR. Another idea is to produce the result object

and then signal a TYPE-ERROR that this object doesn't match the

type-specifier for the result type.

Cassels points out that two similar operations are defined in CLtL to be

inconsistent with each other:

(replace (make-array 4) #(1 2 3)) just picks the shortest length, and

"the extra elements near the end of the longer subsequence are not

involved in the operation" so the result is #(1 2 3 NIL)

#4(1 2 3) duplicates the last element, so it's like #(1 2 3 3)

#2(1 2 3) "is an error".


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