Difference between revisions of "User:WillNess"
Jump to navigation
Jump to search
(4 intermediate revisions by the same user not shown) | |||
Line 1: | Line 1: | ||
⚫ | |||
− | I'm interested in Haskell. |
||
− | |||
⚫ | |||
<haskell> |
<haskell> |
||
Line 9: | Line 7: | ||
primes = 2 : g (fix g) |
primes = 2 : g (fix g) |
||
where |
where |
||
− | g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) |
+ | g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) [] |
[[x*x, x*x+2*x..] | x <- xs]) |
[[x*x, x*x+2*x..] | x <- xs]) |
||
⚫ | |||
⚫ | |||
⚫ | |||
− | fix g = xs where xs = g xs |
+ | fix g = xs where xs = g xs -- global defn to avoid space leak |
+ | |||
⚫ | |||
⚫ | |||
⚫ | |||
</haskell> |
</haskell> |
||
<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]]. |
<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 |
+ | The constructive definition of primes is the Sieve of Eratosthenes: |
− | |||
⚫ | |||
− | |||
− | where |
||
⚫ | |||
+ | using standard definition |
||
::::<math>\textstyle\mathbb{N}_{k} = \{ n \in \mathbb{N} : n \geq k \}</math>   . . . or,  <math>\textstyle\mathbb{N}_{k} = \{k\} \bigcup \mathbb{N}_{k+1}</math>   :) :) . |
::::<math>\textstyle\mathbb{N}_{k} = \{ n \in \mathbb{N} : n \geq k \}</math>   . . . or,  <math>\textstyle\mathbb{N}_{k} = \{k\} \bigcup \mathbb{N}_{k+1}</math>   :) :) . |
||
− | Trial division sieve: |
+ | Trial division sieve is: |
− | ::::<math>\textstyle\mathbb{T} = \{n \in \mathbb{N}_{2}: (\ |
+ | ::::<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>. |
+ | 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>, but even ancient Greeks knew better. |
Revision as of 16:54, 19 November 2011
A perpetual Haskell newbie. I like this semi-one-liner:
-- inifinte folding idea due to Richard Bird
-- double staged production idea due to Melissa O'Neill
-- tree folding idea Dave Bayer / simplified formulation Will Ness
primes = 2 : g (fix g)
where
g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) []
[[x*x, x*x+2*x..] | x <- xs])
fix g = xs where xs = g xs -- global defn to avoid space leak
gaps k s@(c:t) -- == minus [k,k+2..] (c:t), k<=c,
| k < c = k : gaps (k+2) s -- fused to avoid a space leak
| True = gaps (k+2) t
foldi
is on Tree-like folds page. union
and more at Prime numbers.
The constructive definition of primes is the Sieve of Eratosthenes:
using standard definition
- . . . or, :) :) .
Trial division sieve is:
If you're put off by self-referentiality, just replace or on the right-hand side of equations with , but even ancient Greeks knew better.