I’ll never forget when I first discovered that Smalltalk has no special forms
for conditionals. Smalltalk has no
if statement. That moment changed the way
I think about programming (as happens with any worthwhile language, according
to Alan Perlis).
A conditional expression in Smalltalk is just a simple method call on a boolean object.
(x > 3) ifTrue: [y process]
The square brackets form a lambda expression;
ifTrue: is the name of a
method. Here is the same structure translated into Python syntax.
(x > 3).ifTrue(lambda: y.process())
That all looks fine, if not particularly inspiring. The real “aha!” comes when
you start to ask how such a method could possibly be implemented. If there’s no
if statement, how can a boolean object decide whether or not to run the
callback function? It can’t, but it doesn’t need to. Since there are two
false (each of which is the only instance of a
False), each one can have its own behavior. So
ifTrue: method simply runs the callback, and
false’s is even
True>>ifTrue: consequent ^consequent value False>>ifTrue: consequent ^nil
Again, here is a (ficticious) Python version of the same structure.
class true(bool): def ifTrue(self, consequent): return consequent() class false(bool): def ifTrue(self, consequent): return None
This is the definition of polymorphism – two objects can provide the same interface with different behavior.
If you understand this concept and apply it in your every-day programming, you code will turn out simpler, more concise, and easier to modify or extend. How? Let’s look at a practical example.
class Hash # Merges self with another hash, recursively. # # This code was lovingly stolen from some random gem: # http://gemjack.com/gems/tartan-0.1.1/classes/Hash.html # # Thanks to whoever made it. def deep_merge(hash) target = dup hash.keys.each do |key| if hash[key].is_a? Hash and self[key].is_a? Hash target[key] = target[key].deep_merge(hash[key]) next end target[key] = hash[key] end target end end
This code defines a new binary operation,
Hash instances. It
is like the existing
Hash#merge except that it is defined recursively, so
that when the left- and right-hand entries are both
Hash instances, they are
“deeply merged” instead of one replacing the other.
This code is quite useful and simple enough, but it can be improved. I tend to identify things to improve by the smell; though I don’t usually manage to identify smells consciously, that’s how my mind likes to work.
The first smell I notice here is a “copy, modify, replace” pattern common in
imperative-style writing. Rather than copying and updating structures in-place,
it’s usually clearer and more concise to generate the new structure directly,
so let’s try to make this code more stylistically functional. In this case, we
can simplify things a lot by writing
deep_merge in terms of
class Hash def deep_merge(hash) merge(hash) do |key, lhs, rhs| if lhs.is_a? Hash and rhs.is_a? Hash lhs.deep_merge(rhs) else rhs end end end end
The second (and more important) smell is use of the
is_a? method. Often,
is_a? can be replaced by polymorphism.
deep_merge is recursive, we can usefully revise our definition of
deep_merge to apply to all objects, not just
Hash instances. When we do so,
most applications of
deep_merge are degenerate – if the left-hand side is
Hash, the left-hand object is simply replaced by the right-hand object.
class Object def deep_merge(other) other end end
The remaining cases can now be handled even more simply.
class Hash def deep_merge(other) merge(other) do |key, lhs, rhs| lhs.deep_merge(rhs) end rescue TypeError super(other) end end
(I’ve changed the parameter name here from
other because we are no
longer assuming it is a
Organizing things this way has benefits beyond clarity. Suppose we want to
alter the meaning of
deep_merge so that it concatenates arrays. All we need
to do now is override
deep_merge in the
class Array def deep_merge(other) self + other rescue TypeError super(other) end end
Or suppose instead that we want the elements of the arrays to be deeply merged.
class Array def deep_merge(other) (0...[size, other.size].max).map do |i| if i < other.size self[i].deep_merge(other[i]) else self[i] end end end end
Though there’s some noise in this code to deal with arrays of uneven lengths,
its structure is still straightforward. Further, we didn’t need to touch
Hash#deep_merge to add this feature.