User:WillNess: Difference between revisions

Latest revision as of 13:50, 21 February 2023

Monad is composable computation descriptions.

--   infinite folding due to Richard Bird
--   double staged primes production due to Melissa O'Neill
--   tree folding idea Heinrich Apfelmus / Dave Bayer 
primes = 2 : _Y ((3:) . gaps 5  
                      . foldi (\(x:xs) -> (x:) . union xs) []
                      . map (\p-> [p*p, p*p+2*p..])) 

_Y g = g (_Y g)  -- multistage production via Y combinator

gaps k s@(c:t)                        -- == minus [k,k+2..] (c:t), k<=c,
   | k < c     = k : gaps (k+2) s     --     fused for better performance
   | otherwise =     gaps (k+2) t     -- k==c

foldi is on Tree-like folds page. union and more at Prime numbers.

The constructive definition of primes is the Sieve of Eratosthenes, P = N_₂\N_₂_*N_₂ = N_₂\P_*N_₂ :

S = N_{2} ∖ ⋃_{p \in S} {p q : q \in N_{p}}

using standard definition

N_{k} = {n \in N : n \geq k}

. . . or,

N_{k} = {k} ⋃ N_{k + 1}

.

Trial division sieve is:

T = {n \in N_{2} : (\forall p \in T) (2 \leq p \leq \sqrt{n} \Rightarrow \neg (p ∣ n))}

If you're put off by self-referentiality, just replace $S$ or $T$ on the right-hand side of equations with $N_{2}$ , as the ancient Greeks might or mightn't have done, as well.

@@ Line 1: / Line 1: @@
-I am a newbie, interested in Haskell.
+[https://wiki.haskell.org/index.php?title=Monad&oldid=63472 Monad is composable computation descriptions].
-I like ''[http://ideone.com/qpnqe this]'':
+----
+I like ''[http://ideone.com/qpnqe this one-liner]'':
 <haskell>
-primes = 2 : g (fix g) -- double staged production idea due to M. O'Neill
+--   infinite folding due to Richard Bird
-   where
+--   double staged primes production due to Melissa O'Neill
-    g xs = 3 : (gaps 5 $ foldi (\q:qs -> (q:) . union qs)
+--   tree folding idea Heinrich Apfelmus / Dave Bayer
-                               [[p*p, p*p+2*p..] | p <- xs])
+primes = 2 : _Y ((3:) . gaps 5
+                      . foldi (\(x:xs) -> (x:) . union xs) []
+                      . map (\p-> [p*p, p*p+2*p..]))
-fix g = xs where xs = g xs
+_Y g = g (_Y g)  -- multistage production via Y combinator
-gaps k s@(x:xs) = if k < x      -- | k<=x = minus [k,k+2..] xs
+gaps k s@(c:t)                        -- == minus [k,k+2..] (c:t), k<=c,
-        then k : gaps (k+2) s   -- inlined to avoid a space leak
+   | k < c     = k : gaps (k+2) s     --     fused for better performance
-        else     gaps (k+2) xs
+   | otherwise =     gaps (k+2) t     -- k==c
 </haskell>
-<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]]. More at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]].
+<code>foldi</code> is on [[Fold#Tree-like_folds|Tree-like folds]] page. <code>union</code> and more at [[Prime numbers#Sieve_of_Eratosthenes|Prime numbers]].
+The constructive definition of primes is the Sieve of Eratosthenes, '''P''' &nbsp;=&nbsp; '''N'''<sub><sub>2</sub></sub>\'''N'''<sub><sub>2</sub></sub><sub>*</sub>'''N'''<sub><sub>2</sub></sub> &nbsp;=&nbsp; '''N'''<sub><sub>2</sub></sub>\'''P'''<sub>*</sub>'''N'''<sub><sub>2</sub></sub> :
+::::<math>\textstyle\mathbb{S} = \mathbb{N}_{2} \setminus \bigcup_{p\in \mathbb{S}} \{p\,q:q \in \mathbb{N}_{p}\}</math>
+using standard definition
+::::<math>\textstyle\mathbb{N}_{k} = \{ n \in \mathbb{N} : n \geq k \}</math> &emsp; . . . or, &ensp;<math>\textstyle\mathbb{N}_{k} = \{k\} \bigcup \mathbb{N}_{k+1}</math> .
+Trial division sieve is:
+::::<math>\textstyle\mathbb{T} = \{n \in \mathbb{N}_{2}: (\forall p \in \mathbb{T})(2\leq p\leq \sqrt{n}\, \Rightarrow \neg{(p \mid n)})\}</math>
+If you're put off by self-referentiality, just replace <math>\mathbb{S}</math> or <math>\mathbb{T}</math> on the right-hand side of equations with <math>\mathbb{N}_{2}</math>, as the ancient Greeks might or mightn't have done, as well.