infinity
Infinite sequences for Node.js. Supports typical array functions and more, all evaluated lazily.
npm install infinity
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Version | 0.0.4 last updated 8 months ago |
Keywords | sequence, stream, functional, infinite, filter, range |
Repository | https://github.com/rprieto/infinity.git (git) |
Homepage | |
Bugs | https://github.com/rprieto/infinity/issues |
Dependencies | coffee-script |
infinity.js
Infinite sequences for Node.js
Supports typical array functions and more, all evaluated lazily.
Joint effort of Andrew Jones, Maxim Filimonov, Leonardo Borges and Romain Prieto, during a ThoughtWorks Node.js hack night.
Generators
infinite = require 'infinity'
infinite.range 9 # naturals from 9 to infinity
infinite.cycle [1, 2, 3] # repeats 1, 2, 3 forever
infinite.iteration 2, (i) -> i*2 # all powers of 2
infinite.zip s1, s2, s3 # a steam pairing all given streams 1 to 1
infinite.cons 99, 98, stream # a new stream starting with the given fixed values
infinite.fromArray [3, 6, 8] # this one is actually finite, but still lazy
infinite.primes() # sequence of prime numbers
infinite.fibonacci() # fibonacci sequence
Operations
These operations create a new lazy stream:
stream = infinite.range 1
stream.filter (i) -> i%2 == 0 # a new stream with even numbers only
stream.map (i) -> i*2 # a new stream with all elements doubled
stream.skip 2 # a new stream that starts 2 elements further
stream.takeWhile (i) -> i < 5 # a finite stream that stops at the given condition
And these ones resolve a stream (non lazy):
# get the first 5 elements as a javascript array
stream.take 5
# reduce the stream to a single value
# can only be called on a finite stream (after takeWhile)
sum = (acc, i) -> acc + i
stream.reduce 0, sum
Example
FizzBuzz that doesn't use loops or modulo:
naturals = infinite.range 1
threes = infinite.cycle ['', '', ' fizz']
fives = infinite.cycle ['', '', '', '', ' buzz']
concat = (arr) -> arr.join ''
infinite.zip(naturals, threes, fives).map(concat).take(20)
Custom streams
The basic generators (range, cycle) are not always enough.... but chances are you can express your stream as a head and a tail.
- The head is a function that returns a value
- The tail is a function that returns a stream
For example the following sequence outputs every second item it's given, lazily:
skipper = (s) -> infinite.stream (-> s.first()), (-> skipper s.tail().skip(1))
skipper(infinite.fibonacci()).take 5
# [ 0, 1, 3, 8, 21 ]
This can be used to implement interesting algorithms.
For example, here's the implementation of infinite.primes()
:
primes = ->
sieve = (s) ->
h = s.head()
infinite.stream (-> h), (-> sieve s.tail().filter (x) -> x % h isnt 0)
sieve(infinite.range 2)
infinite.primes().take 5
# [ 2, 3, 5, 7, 11 ]
infinite.primes().filter( (i) -> i > 1000 ).take 5
# [ 1009, 1013, 1019, 1021, 1031 ]
`
Curious about the code?
Go to the folder you just cloned:
npm install
npm test