sieve-0.1.0.1: Sieve is an implementation of the Sieve abstract data type.

Portabilityportable
Stabilityprovisional
MaintainerJohn L. Singleton <jsinglet@gmail.com>
Safe HaskellSafe-Inferred

Data.Sieve

Contents

Description

An implementation of the Sieve abstract data type in Haskell.

A Sieve is a data type with properties analogous to a physical Sieve and it is useful for building up lists of data wherein a specific constraint must be met that cannot be achieved using normal type semantics. A Sieve encapsulates a list that can only hold a certain type, specified by a identity function and is preferable to creating or building up lists by using conditional blocks or by the progressive use of filter. This is especially advantageous if a list is to be passed around and used as an accumulator. In such a configuration, the original declaring type is passed around with the Sieve so that it can be used transparently in subsequent areas of the program.

A Sieve is especially useful for applications that:

  • Will hold onto a list for longer than a single function
  • Need to perform asynchronous stream processing

Consider the following example wherein we wish to create and maintain a list with Integer values greater than 2. 

 f2 :: Sieve Int -> Sieve Int
 f2 s = [7,8,1] ++? s

 f1 :: Sieve Int 
 f1 = let numbersGreaterThanTwo = newSieve (\x -> x > 2) [1,2,3] in f2 $ [4,5,6] ++? numbersGreaterThanTwo

This example produces the list: [7,8,4,5,6,3].

Synopsis

Sieve Backing Types

data Sieve a Source

The type that backs created Sieves.

Constructing a New Sieve

newSieveSource

Arguments

:: (a -> Bool)

A unary function that returns True if this element should be allowed into the Sieve, and False if not.

-> [a]

The initial list to populate this Sieve with. Note that the list will be immediately filtered.

-> Sieve a

The resulting Sieve.

The function newSieve is the preferred method of creating a new Sieve. The sieve that is constructed will immediately filter the list passed as the second parameter to newSieve.

Accessing Pieces of the Sieve

toList :: Sieve a -> [a]Source

The toList function returns the resulting backing list.

relation :: Sieve a -> a -> BoolSource

Returns the backing relation used to determine if a given element is a member of the list.

Building Up Lists With a Sieve

(++?)Source

Arguments

:: [a]

The list you wish to conditionally add to the Sieve.

-> Sieve a

The Sieve to add elements to.

-> Sieve a

The resulting Sieve comprised of the contents of the old Sieve combined with the elements from argument one that met the relation criteria.

The operator for building up lists with a Sieve. The operator should be read as "Conditionally Add." All interaction with Sieve should ideally be through this function/operator. The most basic usage of it can be seen in the following example: 

  f3 =  [0,11,10] ++? ([7,8,9]  ++?  ([4,5,6] ++? newSieve (\x -> x > 2) [1,2,3]))

This produces the list [11,10,7,8,9,4,5,6,3].