Re: TrueClass/FalseClass vs. Boolean

On Sat, 1 Apr 2006, baumanj@xxxxxxxxx wrote:

ara.t.howard@xxxxxxxx wrote:
On Fri, 31 Mar 2006, Christian Neukirchen wrote:
if a.kind_of? Boolean

case y
when Integer
when Boolean
end

What kind of scenarios would you want to use such a construct? It seems to
me not very rubyish to be switching based on the class of an object. As I
understand it, the more conventional way to deal with potentially diverse
argument types is to use the respond_to? method. This keeps the door open
for duck typing.

i generally do use duck typing, but sometimes it is simply not appropriate or
become too verbose. for example, consider a matrix class with the following
behaviour

1d

m[true] #=> all data returned
m[] #=> all data returned
m[true] = elems #=> data along all dims set
m[] = elems #=> data along all dims set
m[int] #=> treat as 1d array, return elem
m[int] = elem #=> treat as 1d array, set idx to elem
m[off, len] #=> treat as 1d array, return elems off thru len
m[off, len] = elem #=> treat as 1d array, set elems off thru len to elem
m[a .. b] #=> treat as 1d array, return elems in range
m[a .. b] = elem #=> treat as 1d array, set elems in range to elem
m[n] #=> idx one matrix by another

2d

generalize all above so multiple indices access in multi-dimensional
fashion. eg

m[0,1,2] #= return idx x,y,z
m[0,30..42,true] #= return column 0, rows 30 to 42, in all height dims

now, in case you are thinking that i'm just being difficult, this is exactly
how the narray class works

http://narray.rubyforge.org/SPEC.en

have fun writing that with respond_to? !! ;-)

also, when working with concrete data structures (like binary output from C
programs) it's often quite sufficient to have a type mapping. further more
it's sometimes critical, for instace writing the word located at offset=42,
len=4 with a packed int value or sending a given sequence of bytes down a
socket that a given type is used and it's much more natural to write


def send buf, which = nil
case which
when Fixnum
socket.write [which].pack('N')
socket.write buf
when NilClass
socket.write [buf.size].pack('N')
socket.write buf
when Range
length = which.last - which.first
socket.write [length].pack('N')
socket.write buf[which]
else
raise TypeError, which.class
end
end

than something using respond_to?.

don't get me wrong - i defintely advocate duck typing, but when the number of
possible input types becomes large and behaviour is different depending on
that type it becomes cumbersome. this is the price we pay for not having c--
style polymorphism. in fact, it's very common to see the two styles combined:

case arg
when Fixnum
...
when Range
...
else # dunno - assume String and use in a duck type fashion
end


another thing a real Boolean class could give is a 'maybe' obj such that
....
although i can't think of anything attm to do with this it would undoubtably
lead to some nice logical constructs that more closely parallel the way we
think.

It sounds like you're talking about a sort of restricted fuzzy logic, which
is very cool, but is not appropriate for the core of a general purpose
language. Considering that every expression can be interpreted in a boolean
context, what would a maybe value do? Either behavior seems wrong.

i disagree - ruby (and hardware) already supports this style of logic in
several ways:

harp:~ > irb
irb(main):001:0> nan = 0.0/0.0
=> NaN
irb(main):002:0> nan * 42
=> NaN
irb(main):003:0> nan * 42 + 42.0
=> NaN
irb(main):004:0> obj = Object.new and obj.taint
=> #<Object:0xb75a5fac>
irb(main):005:0> (obj.to_s << "string").tainted?
=> true
irb(main):006:0> ("string" << obj.to_s).tainted?
=> true

there's no good reason, imho, why this style of logical behaviour could not be
part of the logical classes (TrueClass/FalseClass) and operators (and, or,
not).

regards.

-a
--
share your knowledge. it's a way to achieve immortality.
- h.h. the 14th dali lama


.

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