This episode is a direct continuation of latest issue: it gives example of use of the destructuring bind form
let+ introduced there. I also discuss multiple values, unary functions and functions with optional arguments.
There is a feature of Scheme that I never liked, i.e. the fact that
functions (more in general continuations) can return multiple values.
Multiple values are a relatively late addition to Scheme - they entered in the
standard with the R5RS report - and there has always been some opposition
against them (see for instance this old post by Jeffrey Susskind).
I personally see multiple values as a wart of Scheme, a useless complication
motivated by premature optimization concerns.
It is possible to define functions
returning multiple values as follows:
In order to receive the values a
special syntax is needed, and you cannot do things like the following:
> (apply + (return-three-values))
2. &who: apply
3. &message: "incorrect number of values returned to single value context"
4. &irritants: ((1 2 3))
Instead, you are forced to use let-values or other constructs which
are able to accept values:
> (let-values (((x y z) (return-three-values))) (+ x y z))
In this series I will never use functions returning multiple values,
except the ones in the Scheme standard library (this is why I am
forced to talk about let-values). Instead of using multiple
values, I will return a list of values and I will destructure it
with let+. For instance, I will write
> (let+ ((a b) (list 1 2)) (cons a b))
(1 . 2)
> (let-values (((a b) (values 1 2))) (cons a b))
(1 . 2)
let+ is more elegant and more general than let-values:
everything let-values can do, let+ can do too. let+ can
even faster - in some implementations and in some cases.
Here is a benchmark in Ikarus Scheme:
running stats for (repeat 10000000 (let-values (((x y z) (values 1 2 3))) 'dummy)):
276 ms elapsed cpu time, including 0 ms collecting
277 ms elapsed real time, including 0 ms collecting
0 bytes allocated
running stats for (repeat 10000000 (let+ ((x y z) (list 1 2 3)) 'dummy)):
211 ms elapsed cpu time, including 42 ms collecting
213 ms elapsed real time, including 43 ms collecting
240016384 bytes allocated
As you see, let+ takes only 211 ms to unpack a list of three elements
ten million times; let-values would take 276 ms instead.
On the other hand, let+ involves garbage collection
(in our example 24 bytes are allocate per cycle, and thats means 240
million of bytes) and depending on the situations and the implementation
this may cause a serious slowdown. You may find much better benchmarks
than mine in this thread on comp.lang.scheme and you will see that
you can get any kind of results. let-values seems to be slow in
Ikarus and in Ypsilon with the default optimization options; it can be
faster in other implementations, or in the same implementations with
different options or in different releases.
However, those are implementation
The important thing in my view is the conceptual relevance of a language
construct. Conceptually I think the introduction of multiple values
in Scheme was a mistake, since it added nothing that could not be done
better with containers. I think functions should always return
a single value, possibly a composite one (a list, a vector, or anything
else). Actually, I am even more radical than that and I think that functions
should take a single value, as in SML and Haskell.
If you have a minimalistic mindset (as I have) you will recognize that
multiple argument functions are useless since they can be implemented
as unary functions performing destructuring.
Here is a simple implementation of the idea:
(def-syntax (fn (arg ... . rest) body body* ...)
(let+ ((arg ... . rest) x)
(begin body body* ...))))
(def-syntax (define/fn (name arg ... . rest) body body* ...)
#'(define name (fn (arg ... . rest) body body* ...)))
All the functions defined via define/fn take a single argument, a list,
which is then destructured according to the declared structure.
double expects a list with a single element named x; sum expects
a list with a variable number of elements val; sum-2D expects
a two-element lists made of two-element lists named (x1y1) and
(x2y2) respectively. You can easily imagine more complex examples
with deeply nested lists.
It is interesting to notice that Python has the
list destructuring/tuple unpacking functionality built-in:
This is valid Python code in all versions of Python before Python 3.0.
However, in Python 3X this functionality has been removed for lack of use
The advantage of unary functions is that they are easier to compose,
and many functional patterns (including currying described in
episode #14) becomes possible. However, Scheme is not ML or Haskell, so
let us accept functions with multiple arguments and let us take advantage
of them to implement optional arguments. This is the subject of the
A weekness of standard Scheme is the lack of functions with default
arguments and keyword arguments. In practice, this is a minor weakness
since there many libraries implementing the functionality, although in
different ways, as usual. I recommend you to look at SRFI-88 and
SRFI-89 for more context. Here I will implement equivalent
functionality from scratch,
as yet another exercise to show the power of
let+. Let me start from an example, to make clear the intended
functionality. Let me define a function f with optional arguments
(define/opt (f x y (opt (u 1) (v 2)) . rest)
(printf "Required: ~a Optional: ~a Other: ~a\n"
(list x y) (list u v) rest))
Here x and y are required arguments, u and v
are optional arguments and rest are variadic arguments.
If you do not provide an optional argument, its default value is
be used instead, and f behaves as follows:
> (f 'a 'b 'c 'd 'e 'f)
Required: (a b) Optional: (c d) Other: (e f)
> (f 'a 'b 'c)
Required: (a b) Optional: (c 2) Other: ()
> (f 'a 'b)
Required: (a b) Optional: (1 2) Other: ()
> (f 'a)
2. &who: apply
3. &message: "incorrect number of arguments"
4. &irritants: (#<procedure f> 1)
It is clear that in order to implement the functionality the trick
is to override the defaults of the optional argument with the passed
arguments, if any. To this aim we will need the following helper
I should notice that strictly speaking you do not need a full
restructuring form to implement opt-lambda: since override-with-args
returns a flat list, a form which is able to destructure flat list is
enough. You could implement it easily enough:
However let+ subsumes the functionality of let- and I do not
see the point of introducing yet another binding form, except for sake
of the exercise. Strangely enough, let- looks even slower than let+
running stats for (repeat 10000000 (let- (x y z) (list 1 2 3) 'dummy)):
324 ms elapsed cpu time, including 16 ms collecting
324 ms elapsed real time, including 22 ms collecting
240004096 bytes allocated
You're quite right to say that multiple values are the dual of multiple arguments, and if you disapprove of the latter, it's not surprising that you disapprove of the former. But if you accept multiple arguments and CPS, then multiple values are just passing multiple arguments to the continuation.
The two main irritants around multiple values are that unlike arguments there is no way to name them individually within a function, and <i>a fortiori</i> there is no way for a compiler to determine whether a function returns a fixed number of values or even how many it returns (short of interprocedural analysis). That winds up creating run-time inefficiency even in CPS-converting compilers like Chicken.
Multiple values are old in programming: call-by-result, and for some purposes call-by-reference, are equivalent to multiple values. Ada's <code>out</code> and <code>in out</code> modes are a fairly modern example.