Changes http://wiki.axiom-developer.org/OverloadProb/diff
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**Type Conversion and Overloading problems**
*(Found while developing Biquaternion calculus
support function collection)*
D. Cyganski - July 11-13, 2007
Several non-intuitive problems with overloading and type conversions while
developing the biquaternion support function collection. I have extracted the
minimum code set to illustrate each of these herein.
**Implicit and Explicit Type Conversions**
We begin by illustrating function calling with variously
typed arguments and conversions which we will break
in various ways, some understandable, others not(?), below.
The cos function will produce float outcomes for float arguments
\begin{axiom}
cos(1.237)
\end{axiom}
can handle expressions that mix floats and integers
\begin{axiom}
cos(1.237/2)
\end{axiom}
but will respect an integer expression, as we would want it too, by not
evaluating
\begin{axiom}
cos(2/3)
\end{axiom}
We can coerce the evaluation as a float by forcing the
floating point evaluation of the division and typing of the outcome
in a variety of ways. Each of the following forms is effective in some
appropriate and understandable way. Some act explicitly on the "/"
operator to force a polymorphic choice, others convert the type of
the second constant in each expression with then results in a
proper implicit selection of which "/" definition to use:
\begin{axiom}
cos(2/3::Float)
cos((2/3)::Float)
cos(2/3$Float)
cos((2/3)$Float)
cos(2/[EMAIL PROTECTED])
cos((2/3)@Float)
\end{axiom}
But, as we would expect, it is too late to attempt coercion
after the fact:
coercion operates "on the surface and not deeply"
as illustrated here
\begin{axiom}
cos(2/3)::Float
\end{axiom}
However, there is a real need for a deep coercion
operator that operates on the inner most atomic constants! Suppose we define:
\begin{axiom}
cosf(x:Expression Integer):Expression Integer == 1+cos(x/2)
\end{axiom}
which is an example of a simple function that might be defined in the course
of typical work. We wish to declare functions as having Integer based
arguments and outcomes because this results in behaviors that
preserve our representation of Integer fractions, rather than forming
approximate decimal expansions, which is perferred for purposes of
analytic examination and simplification for both the human and the
axiom system. The axiom book and online resources are full of examples
in which this choice has been made by the authors thanks to the power
of this form of expression - even though it amounts to lying to axiom
in many cases as to the ultimate destiny of the function being defined.
But woe to us if we wish later to evaluate it in a more general way
because it is a tangled web we weave when we practice to decieve:
\begin{axiom}
cosf(2/3)
cosf((2/3)::Float)
\end{axiom}
Thus in effect once we wrap a function around an Integer base definition, we are
stuck and unable to evaluate it as a float later, unlike the core basic
functions that
can be used either way. This forces us to choose the Float type throughout
at a loss of comprehensibility and analyzability, unless we seek to more than
double
our development type by supplying an overloaded Integer base and Float base
version of *every step* of a sequential development of a formula.
Bizarrely, the draw function seems to have the power to override the type
problem as shown here!
\begin{axiom}
draw(cosf(x),x=0..15)
\end{axiom}
Why can't we grant this deep coercion power to some new form of floating
point conversion operation which can be applied at will??? If draw has
this power, why not put it in the hands of the user?
Alternatively,
it would be best to have a *mixed* type - mixed = Interger/Float. Like
Maple expressions it would leave Integers as integers and floats as floats,
unmolested and treated as generic constant quantities will distinguishable
parts until an *evalf* like function that would force them entirely into
the Float type. For example, in Maple, "cos(2/3)+1.2323" remains as is,
while in Axiom we get
\begin{axiom}
cos(2/3)+1.2323
\end{axiom}
In a way, Axiom already has a quantity treated like this - the constant
%pi is treated as a special float which remains unevaluated and does
not force combination of itself with an Integer and simply results
in a new kind of Integer expression of type Pi.
\begin{axiom}
3/4+%pi
\end{axiom}
**Overloading problems**
Now let's examine properties and problems with overloading.
Define the type Q of Hamiltonian biquaternions
\begin{axiom}
C:=Complex Expression Integer
Q:=Quaternion C
\end{axiom}
While developing the support functions, this definition of biquat division
was introduced to simplify the format of the formulae
\begin{axiom}
((x:Q)/(y:Q)):Q == x*inv(y)
\end{axiom}
But is this typed function in any way actually restricted to quaternions?
On the face, it would appear all is normal, here's an example of integer
division
\begin{axiom}
x:=15/6
\end{axiom}
But though the answer was right, the type is now a biquat.
If we don't notice this, and procede, some things seem still to
act normally, for example, no complaint from axiom with
\begin{axiom}
cos(x)
\end{axiom}
Of course we still get a correct answers with
\begin{axiom}
cos(1.237)
\end{axiom}
But let's try to apply this is a simple mixed float/integer function
\begin{axiom}
cos(15.457/6)
\end{axiom}
Obiously the quaternion version of "/" is being invoked despite mismatches
of the arguments and the supposed overloading in effect.
Well, what if we built a new cosine function that forced the form of
of the arguments into certain types to avoid the mismatch?
\begin{axiom}
c(y:Float):Float==cos(y)
\end{axiom}
At first this seems to work, we can still evaluate a float
\begin{axiom}
c(1.237)
\end{axiom}
and we can even get a float answer when we introduce the integer
coercable biquat variable value generated above!!!
\begin{axiom}
c(x)
\end{axiom}
But that was only misdirection, because this breaks down for reasonable
expressions
because of the "/" operation still not being resolved correctly.
\begin{axiom}
c(1.237/2)
\end{axiom}
Rather than complaining about it, what if we tried the various coercions that
served to solve the similar type conversion problem we had when just dealing
with Integer Fraction versus Floats at the top of this page. Our results are
mixed!
Recall that each of the following worked in the previous case, producing the
correct
floating result in each case:
\begin{axiom}
cos(2/3::Float)
cos((2/3)::Float)
cos(2/3$Float)
cos((2/3)$Float)
cos(2/[EMAIL PROTECTED])
cos((2/3)@Float)
\end{axiom}
Try these examples with our type constrained function, which has better luck now
\begin{axiom}
c(2/3::Float)
c((2/3)::Float)
c(2/3$Float)
c((2/3)$Float)
c(2/[EMAIL PROTECTED])
c((2/3)@Float)
\end{axiom}
Could the above problems been avoided by not assigning types to the
function we defined? Let's repeat the entire above example with this
single change for the function c2
\begin{axiom}
c2(y)==cos(y)
c2(1.237)
c2(x)
\end{axiom}
But that was only misdirection, because this breaks down for reasonable
expressions
\begin{axiom}
c2(1.237/2)
\end{axiom}
and various attempts at coercion also fail-compare these results to the
previous ones
\begin{axiom}
c2(2/3::Float)
c2((2/3)::Float)
c2(2/3$Float)
c2((2/3)$Float)
c2(2/[EMAIL PROTECTED])
c2((2/3)@Float)
\end{axiom}
Lastly, we cannot now use the graph function, draw, on such a function
since the wrong / function is used, contrary to the bypassing of internal
types we saw take place with draw in the example prior to the introduction
of operator overloading
\begin{axiom}
draw(c(x),x=0..15)
\end{axiom}
*Not safe at any speed:*
Most oddly, the ordinary cos() function which exposes no "/" division
Now fails to work with draw despite the fact that we just saw it above
still working with Integer and Float arguments applied directly!
\begin{axiom}
draw(cos(x),x=0..15)
\end{axiom}
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forwarded from http://wiki.axiom-developer.org/[EMAIL PROTECTED]
