Hello,
I would like to announce the first release of retrograde, a small Clojure
DSL that deals with the problem of accessing information and results from
previous iterations during the **lazy** transformation of a sequence or
during the **lazy** generation of an infinite sequence. The main motivation
for retrograde was the observation that in tricky cases, when a calculation
requires access to previous iterations, Clojure programmers (the author at
least!) tend to resort to using loop/recur, therefore losing laziness. Even
if one sticks to lazy-seq, handling the passing of various bits of "state"
to the next iteration is awkward at best, with the most horrific cases
including use of real state such as atoms etc.
Retrograde allows access to the results of previous iterations using the
"prime" syntax. For example, here is an infinite list of Fibonacci numbers:
(retrograde/calculate [x 1 (if (and x' x'') (+ x' x'') 0)])
x is defined as the name of the result in each iteration. The result of the
previous iteration then becomes available as x' (read as "x-prime") and the
result of the iteration before that is available as x''. You can go back as far
as you like by adding more prime symbols (single quotes) to the name of your
result.
The optional 1 value after the name of the result is the value of x' during the
first iteration. This is followed by the code fragment that calculates the
value of x in each iteration.
The code produced by the macro uses lazy-seq to make sure that the calculation
happens in a lazy manner, without you having to handle the passing of "state"
to the next iteration.
Retrograde also supports access to previous iterations via accumulators. Let's
look into the (made up) problem of lazily transforming a sequence of numbers so
that each output element is the input number added to all the odd numbers that
have occurred in the sequence so far:
> (retrograde/transform
[odds [] (if (odd? x) (conj odds' x) odds')
x (apply + (conj odds' x))]
(range 10))
(0 1 3 4 8 9 15 16 24 25)
Leiningen dependency: [retrograde "0.9"]
Github: https://github.com/stathissideris/retrograde
Feeback in any form is more than welcome, please let me know.
Thanks,
Stathis
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