How to duck type? - the psychology of static typing in Ruby

[Tim Bates]

Hi all,
Following a discussion in #ruby-lang, I have a suggestion about how to
approach Duck Typing. Below is my dissertation on the subject. My
intention is to incorporate any comments people might have into the text
and then place it on the Wiki as an introduction to Duck Typing for the
static typist.

For those not in on the secret, the idea is that if an object walks like
a duck and quacks like a duck, it may as well be a duck - this being a
metaphor for an arbitrary object that may not be exactly the same class
your code was expecting, but still behaves the same way - see [1] if you
don't follow.

---

Many people coming to Ruby from a statically-typed language are somewhat
afraid of Ruby's dynamism, or "don't get it(TM)". David Black and I
believe that this is in part because it is thought that the uncertainty
and changeability built into Ruby are dangerous and one wants to find
shelter from them.

Please bear with me while I describe some of the possible approaches.

1) People with a Static Typing background often have the urge to do
something like this:

attr_reader :date
def date=(val)
raise ArgumentError.new("Not a Date") if val.class != Date
end

This is not duck typing - this is trying to get Ruby to do Static Typing.

2) Okay, you say, if that's not duck typing, let's do duck typing by
accepting a whole bunch of different input formats and trying to turn
them into something we know how to deal with, like this:

def date=(val)
class="keyword">case val
when Date
@date = val
when Time
@date = Date.new(val.year, val.month, val.day)
when String
if val =~ /(\d{4})\s*[-\/\\]\s*(\d{1,2})\s*[-\/\\]\s*(\d{1,2})/
@date = Date.new($1.to_i,$2.to_i,$3.to_i)
else
raise ArgumentError, "Unable to parse #{val} as date"
end
when Array
if val.length == 3
@date = Date.new(val[0], val[1], val[2])
end
else
raise ArgumentError, "Unable to parse #{val} as date"
end
end

This "normalization" approach has the advantage that the date attribute
getter will always return a Date (producing certainty), but the setter
can take input in a variety of formats.

2.a) Discussing this on #ruby-lang, David Black suggested the following
optimization:

def date=(val)
begin
@date = Date.new(val.year, val.month, val.day)
rescue
begin
val =~ /(\d{4})\s*[-\/\\]\s*(\d{1,2})\s*[-\/\\]\s*(\d{1,2})/
@date = Date.new($1.to_i,$2.to_i,$3.to_i)
rescue
begin
@date = Date.new(val[0], val[1], val[2])
rescue
raise ArgumentError, "Unable to parse #{val} as date"
end
end
end
end

This has the advantage over (2) that it doesn't depend upon the class of
val - if it acts enough like a string to use the =~ operator, then that
clause will handle it, even if it's not descended from String - unlike
the previous example. This makes it "more duck-typed", but still
addresses the static-typist's fear of uncertainty and dynamism by
providing a predictable response from #date (it will always be a Date).
Unfortunately it's also slow.

3) Even "more duck-typed" is the approach of just testing that it
responds to the appropriate methods, like so:

# Accepts an object which responds to the +year+, +month+ and +day+
# methods.
def date=(val)
[:year, :month, :day].each do |meth|
raise ArgumentError unless val.responds_to?(meth)
end
@date = val
end

In this case, we have removed the normalization instituted in example
(2), but we have still ensured that the #date attribute conforms to some
sort of interface, providing certainty. It is now the caller's
responsibility to make sure what they pass fits the [:year, :month,
:day] specification - but this responsibility is documented. However,
this approach violates the Don't Repeat Yourself principle - both the
code and the comment contain the specification, and are not therefore
guaranteed to be in sync.

This approach is what many people believe to be embodied by "Duck
Typing". Given an object, we're checking whether it walks and quacks
like a duck; we're not forcing our caller to use a particular class,
like example (1), but we are forcing our caller to put the data in a
format we can understand, unlike (2) which attempts to deal with every
possible representation of a date, causing volumes of maintenance work -
imagine trying to write a normalization routine like that for every
attribute of every class! In this way, we are moving the responsibility
of putting the data into a reasonable format to the caller, who knows
what format their data is in, from the receiver, who has to guess at
every possible format the caller might send them.

4) The fourth and final approach, which I believe to be the Zen of Duck
Typing, is as follows:

# Accepts an object which responds to the +year+, +month+ and +day+
# methods.
attr_accessor :date

"What?" I hear you cry. "There's no checking there at all! You could
pass it anything!" Yes, gentle reader, but why would you? After all, the
documentation for this method is exactly the same as the one above. If
the programmer using this method does what the documentation says then
the class's behaviour is exactly the same. If they hand it the wrong
thing (accidentally, we assume) then the only difference is that it
breaks when the setter is called, rather than some time after the getter
is called and we try and call a non-existent method on the result.

A common response to this often contains the phrase "meaningless error
messages", but the results of such a mistake are usually, if not always,
far from meaningless. For the most part, they look something like this:

NoMethodError: undefined method `year' for "notadate":String

This tells me a lot: namely, that some part of my code (whose location
is given in the subsequent backtrace) expected "notadate" to have a
:year method, and it didn't. From this it is fairly trivial to deduce
that something, somewhere, has fed the wrong thing to the date= setter
method. Chances are that if your code is well-factored, there aren't a
whole lot of places that set the date, and the location of the error can
be found through a little judicious testing; you've lost the certainty
and immediacy of the inline check, but not by much, and you've gained
the flexibility of dynamic typing, and a whole lot less code to maintain.

Now if you'd been writing and collecting unit tests as you went along,
instead of

NoMethodError: undefined method `year' for "notadate":String

you would be seeing

1) Failure:
test_stuff(MyClassTest) [./test/myclasstest.rb:13]:
<false> is not true.

which makes the error even easier to find: you go to test/myclasstest.rb
and see something like:

10: def test_date
11: @obj = Foo.new
12: @obj.date = MyClass.new.notadate
13: assert(@obj.date.respond_to?year))
14: end

and now the error is trivial to trace - the moral of the story being
that when Duck Typing, do your checking in your unit tests, rather than
in the live code. Type errors such as this one are usually the least
common and easiest to trace of errors; if the attribute's documentation
specifies what it is supposed to be, as in the example above, and the
callers of both the getter and the setter methods make no assumptions
about any more or less than what the documentation says, then apart from
keyboarding accidents this will never be a problem.

At [1], Dave Thomas describes Duck Typing as "a way of thinking about
programming in Ruby." I think he means to go a step further than that -
Duck Typing is the _best_ way of thinking about programming in Ruby, and
possibly the _only_ way; as David Black puts it:

"I think the concept of duck typing needs to be supplemented and
expanded on. if, as seems to be the case, Dave thinks of it as a
component of programming style, then it doesn't address language design
itself. As long as duck typing is viewed as a stylistic choice, rather
than a radical language principle, the door is always open to people
saying 'I don't do duck typing', by which they usually mean that they
use kind_of? a lot... of course Ruby itself *does* do duck typing,
whether a given programmer thinks they're doing it or not."

Using kind_of? (or responds_to?) a lot isn't "not doing Duck Typing",
it's simply adding in at run time the kinds of checks that Statically
Typed languages do at compile time, in a usually verbose and necessarily
incomplete fashion.

Rather than trying to make Ruby do Static Typing because one is from a
Static Typing background and that's what one is comfortable with, one
should become comfortable with the dynamic nature of Ruby instead. I
have found that once I stopped assuming that the callers of my method
(who may well be me, in five minutes time, or some user of my library on
the other side of the planet) are stupid and don't know how to read my
documentation (you did write some, didn't you?) then writing in Ruby
became a whole lot more natural and somewhat less verbose. The unit
tests took care of the psychological need to check, somewhere, that the
method was getting passed the right thing, but in reality the whole
debacle is a non-issue; type errors are the most trivial of bugs.

And if you're still worried about that date example, an alternative
solution is this:

def set_date(year, month, day)
@date = Date.new(year, month, day)
end

which, if year, month and day are not numeric, will catch the problem
straight away - without resorting to Static Typing or some approximation
of it. And the way it catches it is telling:

irb(main):027:0> Date.new(2004.0, Rational(12,2), "17")
ArgumentError: comparison of String with 0 failed
from /usr/lib/ruby/1.8/date.rb:560:in `<'
from /usr/lib/ruby/1.8/date.rb:560:in `valid_civil?'
from /usr/lib/ruby/1.8/date.rb:590:in `new'
from (irb):27

This is not "ArgumentError: parameters must be numbers" - the error is
discovered when the Date class attempts to compare that parameter to
zero and can't do it, after assuming that it was valid. And it didn't
make the mistake any harder to find, did it? Notice that it didn't balk
at Floats or Rationals, and with no extra coding from the implementor;
Floats and Rationals look, and quack, like numbers. That's Duck Typing
in action.

[1] http://rubygarden.org/ruby?DuckTyping

 

[ts]

T> "What?" I hear you cry. "There's no checking there at all! You could
T> pass it anything!" Yes, gentle reader, but why would you?

[ruby-talk:99351]
[ruby-talk:99370]


Guy Decoux

 

[Simon Strandgaard]

Broken record time:

The problem with all of these solutions is that they are discovered at
run-time. I find it increasingly irritating when I have to debug
typing errors by running an application that takes some time to get to
the error. Enough of those times, the error is a typing error, so that
I've been harping lately about wanting a duck-type checker hooked in to
"ruby -c".

I do plenty of unittesting, and only rarely have such kind of problems.
I can recommend verbose testing.

 

[John Carter]

Below is my dissertation on the subject. My
intention is to incorporate any comments people might have into the text
and then place it on the Wiki as an introduction to Duck Typing for the
static typist.

Excellent idea, Excellent article. Thanks.

1) People with a Static Typing background often have the urge to do
something like this:

attr_reader :date
def date=(val)
raise ArgumentError.new("Not a Date") if val.class != Date
end

This is not duck typing - this is trying to get Ruby to do Static Typing.

Well, a more sophisticated Static Typer would do...
attr_reader :date
def date=(val)
raise ArgumentError.new("Not a Date") if val.kind_of? Date
end

2.a) Discussing this on #ruby-lang, David Black suggested the following
optimization:

def date=(val)
begin
@date = Date.new(val.year, val.month, val.day)
rescue
begin
val =~ /(\d{4})\s*[-\/\\]\s*(\d{1,2})\s*[-\/\\]\s*(\d{1,2})/
@date = Date.new($1.to_i,$2.to_i,$3.to_i)
rescue
begin
@date = Date.new(val[0], val[1], val[2])
rescue
raise ArgumentError, "Unable to parse #{val} as date"
end
end
end
end

Really cute. More polymorphic, probably a bit slow as exception paths
are usually expected to be "rare" and hence under
optimized. (Certainly true in C++, I don't know about Ruby.) Anybody
care enough to benchmark this?

I would also prefer it to rescue just a No Method exception than just
any Exception. You can really really really hide some horrible bugs catching
just any exception. Like empty catch blocks in Java, it is a truly
evil practice.

4) The fourth and final approach, which I believe to be the Zen of Duck
Typing, is as follows:

# Accepts an object which responds to the +year+, +month+ and +day+
# methods.
attr_accessor :date

The worse breakage I have seen in a Object Oriented System was in a
language called Actor, where for optimization reasons they had forced
the internal representation of their graphics type into pairs of two byte
integers. This turned a very large, very useful generic 2D geometry
library into something totally useless to me. ie. Never gratiutously
break polymorphism, even if you personally can't think of another use
for this code, somebody else can.

For example, suppose you got a string from a SQL server that was a
"date". But since parsing the string is difficult and expensive to do,
you just don't. You stuff it into the date= method and lo and behold,
it just so turns out that for this execution path you _never_ actually
invoke :year, :month, :day, it just travels through your system, and
gets converted to a string with .to_s and written back to SQL.

If you had put additional checks in the code, it would have broken,
and you would have been forced to do an expensive and useless
conversion.




John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : (e-mail address removed)
New Zealand

The universe is absolutely plastered with the dashed lines exactly one
space long.

 

[gabriele renzi]

il Mon, 17 May 2004 22:52:22 +0900, Tim Bates <[email protected]> ha
scritto::

2) Okay, you say, if that's not duck typing, let's do duck typing by
accepting a whole bunch of different input formats and trying to turn
them into something we know how to deal with, like this:

<snipall>

this was an interesting reading, thanks for it.
But It happens to me that I'm locked in a mental loop.
From time to time I think:
- I'll use duck typing.
- I don't need to do that strange things, I'll write a
date_from_string method
- eh, multi methopd dispatch would be nice..
- well, duck typing is actually cheking an interface. Why don't we
just use static checked interfaces? what's wrong with a Date-able
mixin ?
- oh, static sux, I'll use duck typing..

 

[Tim Bates]

T> "What?" I hear you cry. "There's no checking there at all! You could
T> pass it anything!" Yes, gentle reader, but why would you?

[ruby-talk:99351]
[ruby-talk:99370]

For those who can't be bothered finding those mails, they both refer to
some of the security issues involved in assuming that callers give you
what you expect; perhaps I didn't make this clear in my article, but I
am assuming whoever is writing code alongside mine is non-malicious, and
are not deliberately trying to break my code. On the other hand, if they
are malicious, static typing isn't going to gain you very much, you will
still have to validate any and all input. This issue has very little to
do with they typing model used.

Tim.

 

[Gavin Sinclair]

Now if you'd been writing and collecting unit tests as you went along,
instead of

NoMethodError: undefined method `year' for "notadate":String

you would be seeing

1) Failure:
test_stuff(MyClassTest) [./test/myclasstest.rb:13]:
<false> is not true.

which makes the error even easier to find: you go to test/myclasstest.rb
and see something like:

10: def test_date
11: @obj = Foo.new
12: @obj.date = MyClass.new.notadate
13: assert(@obj.date.respond_to?year))
14: end

Now I'm no unit testing guru (I try...) but that test to me seems so
trivial as to be pointless. Since the code it's testing does
absolutely nothing other than assignment, you're not testing the
*code*, you're testing one possible input value.

I think it would be more convincing to talk about unit tests covering
the correct overall operation of the class under scrutiny, rather than
examining some lowly accessor.

# But your psychological needs could differ from mine

The rest of the article's good, though.

Cheers,
Gavin

 

[Tim Bates]

Well, a more sophisticated Static Typer would do...
attr_reader :date
def date=(val)
raise ArgumentError.new("Not a Date") if val.kind_of? Date
end

Ah, of course. But seeing as I never actually do this in code, I can be
excused, right? Thanks for the suggestion.

I would also prefer it to rescue just a No Method exception than just
any Exception. You can really really really hide some horrible bugs catching
just any exception. Like empty catch blocks in Java, it is a truly
evil practice.

Again, you're correct and I hadn't thought much about these pieces of
code beyond the basic functionality since they were throw-away examples
anyway. Although there are cases where the code might throw an
ArgumentError instead of a NoMethodError and still work. The logic
behind catching just any exception was that if one method doesn't work,
for whatever reason, bail out and try a different one...

The worse breakage I have seen in a Object Oriented System was in a
language called Actor, where for optimization reasons they had forced
the internal representation of their graphics type into pairs of two byte
integers. This turned a very large, very useful generic 2D geometry
library into something totally useless to me. ie. Never gratiutously
break polymorphism, even if you personally can't think of another use
for this code, somebody else can.

I'm sorry, I don't quite follow you; can you clarify a little? How did
changing the internal representation break things? How does this relate
to polymorphism or Duck Typing?

For example, suppose you got a string from a SQL server that was a
"date". But since parsing the string is difficult and expensive to do,
you just don't. You stuff it into the date= method and lo and behold,
it just so turns out that for this execution path you _never_ actually
invoke :year, :month, :day, it just travels through your system, and
gets converted to a string with .to_s and written back to SQL.

This is rather risky; you're assuming less than what the documentation
says, and hoping that nobody else assumes any more than you do. That's
just asking for someone to come along and add something that does use
one of those methods, and suddenly things come crashing down around you.
On the other hand, however, if you know what you're doing then this is
one of the advantages of Duck Typing, and part of the beauty of Ruby -
if you *know* that those methods will never get called, the language
never forces you to convert the date into that format if you don't want to.

Tim.

 

[Aredridel]

This is rather risky; you're assuming less than what the documentation

says, and hoping that nobody else assumes any more than you do. That's
just asking for someone to come along and add something that does use
one of those methods, and suddenly things come crashing down around you.

Nah, it crashes down on /them/. Garbage in, garbage out.

Imagine how useful the unix grep command would be if it assumed lines
were 80 characters, and errored if you passed it otherwise.

Static vs dynamic typing is a similar design choice: Let the user put in
whatver they want, and if they get results they don't like, let them
adjust.

Doesn't always apply -- you still validate things where appropriate, but
static typing seems an arbitrary limit to me, not application-specfic
where it's justified.

 

[Tim Bates]

Now I'm no unit testing guru (I try...) but that test to me seems so
trivial as to be pointless. Since the code it's testing does
absolutely nothing other than assignment, you're not testing the
*code*, you're testing one possible input value.

You're right, that's a really stupid example, and I was hoping nobody
would spot it. I tried to think of a better example, and failed,
since my original example (`attr_accessor :date`) is so trivial as to
not need unit testing. (Although the unit testing gurus may disagree
with me, I tend not to write tests for attributes unless I have written
extra code in them - I just assume that attr_accessor does what it's
supposed to.)

I think it would be more convincing to talk about unit tests covering
the correct overall operation of the class under scrutiny, rather than
examining some lowly accessor.

That's right, and the problem arises because the whole article revolves
around writing an accessor method - partly because it's simple, and
partly because that was what I was trying to write when I started the
discussion on #ruby-lang. If you can think of a better example, I'll
happily rewrite it.

# But your psychological needs could differ from mine

No, they don't. You're absolutely correct, that unit test is so trivial
as to be pointless. I don't bother to test attr_accessor either.

The rest of the article's good, though.

Thanks.

Tim.

 

[John Carter]

I'm sorry, I don't quite follow you; can you clarify a little? How did
changing the internal representation break things? How does this relate
to polymorphism or Duck Typing?

The tighter you constrain the interface, the less reusable it is. In the
example I mentioned, the programmers had ruined the polymorphic behaviour
of the library by internally squashing everything into two byte int's. If
they hadn't done that, polymorphism would have allowed the library to work
on _anything_ that inherited from Number. Including the floating point
types I needed.

Duck typing is an even freer form of polymorphism and in the case of the
example I mentioned, with Duck Typing, the geometry library would work on
anything that responded to +,-,*,/,==,<, including Vectors and Matrices!

I'm hinting at a prime rule of polymorphic / duck typed design. Don't
gratuitously constrain the interface. Reusable means you don't know what
other types may be fed to you in future, and to be reusable you must allow
it.

This is rather risky; you're assuming less than what the documentation
says, and hoping that nobody else assumes any more than you do. That's
just asking for someone to come along and add something that does use
one of those methods, and suddenly things come crashing down around you.

True, YAGNI. And when and if it hits you, you also have the option of
catching the exception, performing the conversion and retrying.

On the other hand, however, if you know what you're doing then this is
one of the advantages of Duck Typing, and part of the beauty of Ruby -
if you *know* that those methods will never get called, the language
never forces you to convert the date into that format if you don't want to.

The longer I program in Ruby, the less I like the many other languages I
have used.



John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : (e-mail address removed)
New Zealand

The universe is absolutely plastered with the dashed lines exactly one
space long.

 

[Gavin Sinclair]

You're right, that's a really stupid example, and I was hoping nobody
would spot it. I tried to think of a better example, and failed,
since my original example (`attr_accessor :date`) is so trivial as to
not need unit testing. (Although the unit testing gurus may disagree
with me, I tend not to write tests for attributes unless I have written
extra code in them - I just assume that attr_accessor does what it's
supposed to.)


That's right, and the problem arises because the whole article revolves
around writing an accessor method - partly because it's simple, and
partly because that was what I was trying to write when I started the
discussion on #ruby-lang. If you can think of a better example, I'll
happily rewrite it.

Well, there's the problem. There's no context for the attribute, so
conjuring an example of a meaningful unit test is difficult. [1] If you
want an example for the unit test, you need an example for the class
surrounding the accessor. That would probably enhance the article anyway.

Assuming we don't have this magic example, I propose you leave out the bad
one, and focus instead on arguing the point: that good unit tests

* raise confidence in the ability of the class to perform its function,
rather than focusing on the minor details

* demonstrate the behaviour of the class under the full range of normal
("acceptable") input, and thus serve as practical documentation to an
extent

But such an argument would obviously benefit from a good example

Cheers,
Gavin

[1] I've yet to read an example of unit testing that isn't fairly
convoluted anyway - references, anyone?

 

[Gavin Sinclair]

Duck typing is an even freer form of polymorphism and in the case of the
example I mentioned, with Duck Typing, the geometry library would work
on anything that responded to +,-,*,/,==,<, including Vectors and
Matrices!

I'm hinting at a prime rule of polymorphic / duck typed design. Don't
gratuitously constrain the interface. Reusable means you don't know what
other types may be fed to you in future, and to be reusable you must
allow it.

This is a good guideline for basic types like numbers, strings, arrays,
etc., because people do tend to create or reuse "similar types" (e.g.
DBI::Row or whatever it is, instead of Hash) which should "just work" with
your library.

But as you move away from basic types, through to middle-ground types,
like an XML document representation, to domain types, like a customer, the
balance changes. The prime rule you mention above becomes less and less
important.

If I'm writing some sort of CRM library that I want to share with the
world and have reused, then I should obviously document my interface well,
and not be overly strict about what I accept. Even so, it's very unlikely
that someone else is going to have a class they're using that's
similar-to-but-not-quite my OpenCRM::Customer class. The benefits of a
duck typing attitude are not likely to take root in this system. But I'm
creating a library that is aimed at reuse, so I should allow for reuse.

Now, one step further: an internal application that is never going to
leave company walls. A system (involving customers, invoices, and
payments, say) that is entirely self-contained, and not aimed at reuse at
all. In this case, my system is designed to work with XYZ::Customer
objects, XYZ::Invoice objects, and XYZ:ayment objects. Furthermore, I
may reasonably use some class-checking code at strategic points. This is
not because I feel unsafe without it; it's to encode the practical
limitations of the software. A line like

expect(customer, XYZ::Customer, payment, XYZ:ayment)

is executable documentation. It reminds you at a glance what you're
dealing with. It gives you some context, some knowledge to build on when
you're editing that method after a few months away from it. And it's
documentation that doesn't go out of date. The fact is, I'm not
needlessly restricting the interface (in any practical sense) by including
the above line. The interface was already restricted because of the
special-purpose nature of the system. (That line is using the
'strongtyping' library, BTW.)

The reason I canvas these examples is to emphasise that isolation (i.e.
not planning for reusability) is not necessarily a software sin. I agree
with the spirit(s) of the duck-typing discussions, but don't want some
impressionable lurker thinking "Hey, I'm not considering reuse; am I
overly restricting the interface?; am I using Ruby correctly?" etc.
There's a spectrum of reusability, and the onus is on the programmer to
work out where they are aiming and to use the appropriate techniques.
That's a blessing and a curse. In other languages, you're forced to
handle all typing issues in a certain way, which is restrictive, but at
least you don't have to think. Personally, I'd rather think.

There's a time and place for everything in Ruby

Cheers,
Gavin

 

[John Carter]

This is a good guideline for basic types like numbers, strings, arrays,
etc., because people do tend to create or reuse "similar types" (e.g.
DBI::Row or whatever it is, instead of Hash) which should "just work" with
your library.

But as you move away from basic types, through to middle-ground types,
like an XML document representation, to domain types, like a customer, the
balance changes. The prime rule you mention above becomes less and less
important.

Now, one step further: an internal application that is never going to
leave company walls. A system (involving customers, invoices, and
payments, say) that is entirely self-contained, and not aimed at reuse at
all. In this case, my system is designed to work with XYZ::Customer
objects, XYZ::Invoice objects, and XYZ:ayment objects.

Hmm. I never _aim_ at reuse, I just don't do anything that would
gratuitously prevent reuse in unexpected manners. Write code that
expects it's output to be the input of some, as yet, unspecified
program.

Do you really need to work with XYZZY::Customer objects, or could your
interface be something much simpler? Perhaps a tuple of numbers? Hey!
Looky I have built graphical output into my program! How?! By making
my output something simple, (tuples instead of XYZZY::Customer) I can
feed it trivially into any graphics package like gnuplot.

If you write things made out of Big Balls of Mud, don't be suprised
when the result glues it self up into an un-reusable Bigger Muddier Ball.

So invoicing passes a big hairy XYZZY::Customer object to Billing,
Billing only uses the name and address, shouldn't you have weakened
the interface so you only pass a name and address to billing instead?

Furthermore, I may reasonably use some class-checking code at
strategic points.

So long as ".kind_of?" is just a short cut for a long list of ".respond_to?"'s

The reason I canvas these examples is to emphasise that isolation (i.e.
not planning for reusability) is not necessarily a software sin.

Never plan on being resuable.

Plan on being simple.

Plan on being seperable.

Plan on being generous on what you accept and rigorous on what you
deliver.

http://www.catb.org/~esr/writings/taoup/html/
http://www.laputan.org/mud/mud.html

John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : (e-mail address removed)
New Zealand

The universe is absolutely plastered with the dashed lines exactly one
space long.

 

[James Britt]

Tim Bates wrote:

At [1], Dave Thomas describes Duck Typing as "a way of thinking about
programming in Ruby." I think he means to go a step further than that -
Duck Typing is the _best_ way of thinking about programming in Ruby, and
possibly the _only_ way; as David Black puts it:

"I think the concept of duck typing needs to be supplemented and
expanded on. if, as seems to be the case, Dave thinks of it as a
component of programming style, then it doesn't address language design
itself. As long as duck typing is viewed as a stylistic choice, rather
than a radical language principle, the door is always open to people
saying 'I don't do duck typing', by which they usually mean that they
use kind_of? a lot... of course Ruby itself *does* do duck typing,
whether a given programmer thinks they're doing it or not."

Using kind_of? (or responds_to?) a lot isn't "not doing Duck Typing",
it's simply adding in at run time the kinds of checks that Statically
Typed languages do at compile time, in a usually verbose and necessarily
incomplete fashion.

Rather than trying to make Ruby do Static Typing because one is from a
Static Typing background and that's what one is comfortable with, one
should become comfortable with the dynamic nature of Ruby instead.

But why?

That's somewhat rhetorical; I'm happier with duck typing, but there
seems to be two issues here. One is understanding what ducking typing
is. The other is understanding what it buys you over static typing.

I don't think there's all that much misunderstanding over the former,
but the latter is a sticking point for some people.

One (possible, and not mine) argument against duck typing is that you
are essentially required to write assorted unit tests beyond checking
for operationally correct behavior; the compilation step for a staticly
typed language offers these (albeit compulsory) "unit tests" for free.

To me, that's the rub. A compiler forces me to write code to pass tests
that may have little bearing on the program's actual operation; the
overhead tends to outweigh the speed of development offered by duck
typing, even factoring in the "extra" unit tests I may have to write.


James

 

[Hal Fulton]

But why?

That's somewhat rhetorical; I'm happier with duck typing, but there
seems to be two issues here. One is understanding what ducking typing
is. The other is understanding what it buys you over static typing.

Well, this is partly a side issue.

Like many people, I learned BASIC first. Then I learned FORTRAN. Then a
smattering of ALGOL. Then Pascal (finally a language with lowercase
letters in its name). Then C. Then Object Pascal, C++, Java. And so on.

I hate to admit it, but all through my career, since high school, I took
it for granted that the elements of an array were all of the same type.

Sure, you can have an array of pointers. You can even do that in C,
which is another way of saying you can do it in assembly language.

But a full-fledged, ordinary, everyday array... with mixed types?
Numbers and objects and strings, oh my!

Of course, my education could have been broader, and probably should
have been.

But imagine my joy when it dawned on me that this was for the
convenience of the compiler writer, not the programmer. That was on
learning Ruby in 1999.

It ain't duck typing. But it is dynamic typing. Would I do without
it again? Only for pay.


Hal

 

[Dave Burt]

[1] I've yet to read an example of unit testing that isn't fairly
convoluted anyway - references, anyone?

The simplest example of unit testing I've seen (except for Tim's contrived
example) is at the start of the Extreme Programming site's Coffee Maker:
http://www.extremeprogramming.org/example/piacode.html

 

[Dave Thomas]

One (possible, and not mine) argument against duck typing is that you
are essentially required to write assorted unit tests beyond checking
for operationally correct behavior; the compilation step for a
staticly typed language offers these (albeit compulsory) "unit tests"
for free.

No one requires you to do this. Instead, you do this if you feel
uncomfortable with your code. After a while, you stop feeling
uncomfortable, and realize that all this type discussion is simply a
red herring, and get back to writing proper code and the tests to test
it. A few months after that, you're surprised when you realize that you
haven't had a single type-related error.

This is simple a question of experience. When you first start out, you
feel exposed because Ruby doesn't let you add type annotations to
variables (I know I did). Knowing that it worked OK in Smalltalk, I
made the conscious decision not to fight this, but instead to see how
it worked just doing it the Ruby way. It was hard at first, and I kept
fighting some inner demon who wanted me to add "kind_of?" calls
everywhere. But after a while, I realized that my code was no less
accurate than it was in Java, and that I didn't really need the type
information. Once I overcame that fear, I suddenly realized that I
could also get a lot of benefit from the flexibility: the realization
that type != class, but rather that type equals capabilities was a
revelation, and it has fundamentally changed the way I code.

However, I honestly believe that there is no short cut on this path:
the only way to come to understand the true nature of typing in Ruby is
to write a bunch of code the Ruby way, and to gain your own experience
(and form your own opinions). Unit tests are a safety net that can be
used while crossing that wire. Once across, you'll be able to form your
own opinions about the effectiveness of unit tests for type-related
checking.


Cheers

Dave

 

[Aredridel]

If I'm writing some sort of CRM library that I want to share with the

world and have reused, then I should obviously document my interface well,
and not be overly strict about what I accept. Even so, it's very unlikely
that someone else is going to have a class they're using that's
similar-to-but-not-quite my OpenCRM::Customer class. The benefits of a
duck typing attitude are not likely to take root in this system. But I'm
creating a library that is aimed at reuse, so I should allow for reuse.

How about a delegator to your Customer class, with some extensions for
distributed computing?

That's a very common one that can be gotten wrong: Will your type checks
work with a dRb proxied object?

Ari

 

[Thomas Fini Hansen]

There's a spectrum of reusability, and the onus is on the programmer to
work out where they are aiming and to use the appropriate techniques.

Let me tell you a little story...

Once upon a time I was stuck at work doing psychological support. It
was an Windows server and hence, 'not my problem', but the guy lived
near my place, so we usually walked home together, so I stuck around.

I figured I might as well do something semi-useful, and had been
irritated by the fact that internal documentation was these HTML files
stuck somewhere. I had an idea about these editable pages, so I
started fiddling with some PHP...

In a few hours I had something that worked, you could edit sections on
a page, add and delete pages, so on and so forth. Didn't look to bad
on the outside, but the code wasn't exactly what you'd call, ahem,
structured. It was a quick hack, and it worked.

I thought, "I'll call this SiDoc", from the initials of the company,
and proceeded to show it to my boss. He liked it, and it was quickly
decided that this was what we'd use for internal documentation. All
was nice.

A little while later, he came and asked if I could make a copy with a
'view only' version, so you'd have 2 pages, where only one could
edit. I said 'sure', made a copy and removed the edit links from one.

It wasn't until a few days later that I discovered that my one day
hack was suddenly the CMS system for a site... Made me wish I'd
actually bothered to clean up the duplicate code.

That was a few years ago. SiDoc is still the internal documentation
system, even though the company has changed name and wikis has gained
popularity. And it's still the CMS behind a site about small local
harbours.

I learned 2 things from this: Don't clean up your code 'later', and
there's no limit on how your code will get abused. Even by
yourself.