Recently, a topic of monads[1] occupied my attention. Since I'm not
very good at writing blogs, I decided to write a library instead.
Monadologie is the result of such effort:
https://github.com/dsrkoc/monadologieThe library is in its early stage of development and if there is
someone who shared my interest in such a subject, I would very much
appreciate a feedback.
I'm posting this on the Groovy mailing list because, although tons of
material about monads lay around the internet, there doesn't seem to
be much related to Groovy in particular (and, as I said, I'm a lousy
blog writer).
Monad comprehension[2] is generalization of list comprehensions[3]
known from other languages. My implementation was heavily influenced
by Scala's for comprehension[4].
As an example, let's calculate all Pythagorean triples[5] (sets of
three integers satisfying Pythagorean theorem) up to a given number:
8<------------------------------
import static hr.helix.monadologie.MonadComprehension.foreach
def pythags(n) {
foreach {
z = takeFrom { 1..n }
x = takeFrom { 1..z }
y = takeFrom { x..z }
guard { x**2 + y**2 == z**2 }
yield { [x, y, z] }
}
}
assert pythags(12) == [[3, 4, 5], [6, 8, 10]]
------------------------------>8
where 'foreach' block denotes the comprehension. 'y' is taken from the
(x..z) range, for each 'x', and 'x' is taken for each 'z'. 'guard'
acts as a filter that lets only values that satisfy given expression
to pass to 'yield'. 'yield' gives result for each step. The final
result is a list of all yields.
[1]
http://en.wikipedia.org/wiki/Monad_(functional_programming)
[2]
http://homepages.inf.ed.ac.uk/wadler/papers/monads/monads.ps (postscript)
[3]
http://en.wikipedia.org/wiki/List_comprehension[4]
http://www.artima.com/pins1ed/for-expressions-revisited.html#23.1[5]
http://en.wikipedia.org/wiki/Pythagorean_tripleThank you for your attention,
Dinko
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