Arne Babenhauserheide <arne_...@web.de> writes:
> Hi,
>
> I just recreated a fluid advection exercise in Guile Scheme and I’m not
> quite happy with its readability. Can you help me improve it?
>
> My main gripe is that the math does not look instantly accessible.
>
> The original version was in Python:
>
> psi[i] - c1*(psi[i+1] - psi[i-1]) + c2*(psi[i+1] - 2.0*psi[i] + psi[i-1])
>
> My port to Scheme looks like this:
>
> (let ((newvalue (+ (- (psir i)
> (* c1 (- (psir (+ i 1)) (psir (- i 1)))))
> (* c2 (+ (- (psir (+ i 1)) (* 2 (psir i)))
> (psir (- i 1)))))))
> (array-set! psinew newvalue i))
Guile supports SRFI-105, so that could be:
{{psi[i] - {c1 * {psi[{i + 1}] - psi[{i - 1}]}}} + {c2 * {{psi[{i + 1}] -
{2 * psi[i]}} + psi[{i - 1}]}}}
which is less readable than the Python version because there's no
first-hand support for operator precedence so we {} group all binary
operations, plus we need to use {} within [], plus we must separate
operators with whitespace, but maybe it's acceptable.
You can put "#!curly-infix" at the start of your Scheme file to enable
SRFI-105 syntax.
Note that all those 'psi[x]' expressions will expand to
($bracket-apply$ psi x)
and you can implement a macro of that name to turn that into whatever is
suitable at compile-time. (If performance is not an issue, SRFI-123
provides a run-time polymorphic procedure for $bracket-access$.)
With a smart definition of $bracket-apply$, you could also cut down
those psi[{i + 1}] to psi[i + 1]. The latter will expand to
($bracket-apply$ psi i + 1)
which could be accommodated for in a special-purpose definition of
$bracket-apply$ such as:
(syntax-rules ()
((_ obj index)
(obj index))
((_ obj x operator y)
(obj (operator x y))))
That would *not* support e.g. psi[i + j - 1], which would instead need
to be written e.g. psi[{i + j} - 1], i.e. we only save one pair of {},
but maybe that's good enough.
By the way, e.g. {x - y + z} will turn into ($nfx$ x - y + z), and maybe
there's a definition for $nfx$ out in the wild (or you can create one)
that does operator precedence. (That could then also be used in
$bracket-apply$.) So optimally, the whole thing could become:
{psi[i] - c1 * {psi[i + 1] - psi[i - 1]} + c2 * {psi[i + 1] - 2 * psi[i] +
psi[i - 1]}}
Taylan
