chimera: Lazy, infinite streams with O(1) indexing.

[ bsd3, library, unclassified ] [ Propose Tags ]

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Versions [faq] 0.2.0.0
Dependencies base (>=4.5 && <5), chimera, ghc-prim, transformers, vector [details]
License BSD-3-Clause
Copyright 2017-2018 Bodigrim
Author Bodigrim
Maintainer andrew.lelechenko@gmail.com
Home page https://github.com/Bodigrim/chimera#readme
Source repo head: git clone https://github.com/Bodigrim/chimera
Uploaded by Bodigrim at Tue Dec 4 00:23:12 UTC 2018
Distributions NixOS:0.2.0.0, Stackage:0.2.0.0
Executables find-foo
Downloads 61 total (29 in the last 30 days)
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Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2018-12-04 [all 1 reports]

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Readme for chimera-0.2.0.0

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chimera

Lazy, infinite streams with O(1) indexing. Most useful to memoize functions.

Example 1

Consider following predicate:

isOdd :: Word -> Bool
isOdd 0 = False
isOdd n = not (isOdd (n - 1))

Its computation is expensive, so we'd like to memoize its values into Chimera using tabulate and access this stream via index instead of recalculation of isOdd:

isOddBS :: Chimera
isOddBS = tabulate isOdd

isOdd' :: Word -> Bool
isOdd' = index isOddBS

We can do even better by replacing part of recursive calls to isOdd by indexing memoized values. Write isOddF such that isOdd = fix isOddF:

isOddF :: (Word -> Bool) -> Word -> Bool
isOddF _ 0 = False
isOddF f n = not (f (n - 1))

and use tabulateFix:

isOddBS :: Chimera
isOddBS = tabulateFix isOddF

isOdd' :: Word -> Bool
isOdd' = index isOddBS

Example 2

Define a predicate, which checks whether its argument is a prime number by trial division.

isPrime :: Word -> Bool
isPrime n
  | n < 2     = False
  | n < 4     = True
  | even n    = False
  | otherwise = and [ n `rem` d /= 0 | d <- [3, 5 .. ceiling (sqrt (fromIntegral n))], isPrime d]

Convert it to unfixed form:

isPrimeF :: (Word -> Bool) -> Word -> Bool
isPrimeF f n
  | n < 2     = False
  | n < 4     = True
  | even n    = False
  | otherwise = and [ n `rem` d /= 0 | d <- [3, 5 .. ceiling (sqrt (fromIntegral n))], f d]

Create its memoized version for faster evaluation:

isPrimeBS :: Chimera
isPrimeBS = tabulateFix isPrimeF

isPrime' :: Word -> Bool
isPrime' = index isPrimeBS