Data.ListLike

Generic operations over list-like structures

Written by John Goerzen, [e-mail]

Please start with the introduction at Data.ListLike.

Introduction

Welcome to ListLike.

This module provides abstractions over typical list operations. It is designed to let you freely interchange different ways to represent sequences of data. It works with lists, various types of ByteStrings, and much more.

In this module, you'll find generic versions of most of the functions you're used to using in the Prelude, Data.List, and System.IO. They carry the same names, too. Therefore, you'll want to be careful how you import the module. I suggest using:

import qualified Data.ListLike as LL

Then, you can use LL.fold, LL.map, etc. to get the generic version of the functions you want. Alternatively, you can hide the other versions from Prelude and import specific generic functions from here, such as:

import Prelude hiding (map)
import Data.ListLike (map)

The module Data.ListLike actually simply re-exports the items found in a number of its sub-modules. If you want a smaller subset of Data.ListLike, look at the documentation for its sub-modules and import the relevant one.

In most cases, functions here can act as drop-in replacements for their list-specific counterparts. They will use the same underlying implementations for lists, so there should be no performance difference.

You can make your own types instances of ListLike as well. For more details, see the notes for the ListLike typeclass.

Creation & Basic Functions

List transformations

Conversions

Reducing lists (folds), from FoldableLL

Special folds

and :: ListLike full Bool => full -> Bool

Returns True if all elements are True

or :: ListLike full Bool => full -> Bool

Returns True if any element is True

sum :: (Num a, ListLike full a) => full -> a

The sum of the list

product :: (Num a, ListLike full a) => full -> a

The product of the list

fold :: (FoldableLL full item, Monoid item) => full -> item

Combine the elements of a structure using a monoid. fold = foldMap id

foldMap :: (FoldableLL full item, Monoid m) => (item -> m) -> full -> m

Map each element to a monoid, then combine the results

Building lists

Scans

Accumulating maps

Infinite lists

Unfolding

Sublists

Extracting sublists

Predicates

Searching lists

Searching by equality

Searching with a predicate

Indexing lists

Zipping and unzipping lists

zip :: (ListLike full item, ListLike fullb itemb, ListLike result (item, itemb)) => full -> fullb -> result

Takes two lists and returns a list of corresponding pairs.

zipWith :: (ListLike full item, ListLike fullb itemb, ListLike result resultitem) => (item -> itemb -> resultitem) -> full -> fullb -> result

Takes two lists and combines them with a custom combining function

unzip :: (ListLike full (itema, itemb), ListLike ra itema, ListLike rb itemb) => full -> (ra, rb)

Converts a list of pairs into two separate lists of elements

Monadic Operations

sequence_ :: (Monad m, FoldableLL full (m item)) => full -> m ()

Evaluate each action, ignoring the results. Same as mapM_ id.

mapM_ :: (Monad m, FoldableLL full item) => (item -> m b) -> full -> m ()

A map in monad space, discarding results.

Input and Output

class ListLike full item => ListLikeIO full item | full -> item where

An extension to ListLike for those data types that support I/O. These functions mirror those in System.IO for the most part. They also share the same names; see the comments in Data.ListLike for help importing them.

Note that some types may not be capable of lazy reading or writing. Therefore, the usual semantics of System.IO functions regarding laziness may or may not be available from a particular implementation.

Minimal complete definition:

• hGetLine
• hGetContents
• hGet
• hGetNonBlocking
• hPutStr

Methods

hGetLine :: Handle -> IO full

Reads a line from the specified handle

hGetContents :: Handle -> IO full

Read entire handle contents. May be done lazily like hGetContents.

hGet :: Handle -> Int -> IO full

Read specified number of bytes. See hGet for particular semantics.

hGetNonBlocking :: Handle -> Int -> IO full

Non-blocking read. See hGetNonBlocking for more.

hPutStr :: Handle -> full -> IO ()

Writing entire data.

hPutStrLn :: Handle -> full -> IO ()

Write data plus newline character.

getLine :: IO full

Read one line

getContents :: IO full

Read entire content from stdin. See hGetContents.

putStr :: full -> IO ()

Write data to stdout.

putStrLn :: full -> IO ()

Write data plus newline character to stdout.

interact :: (full -> full) -> IO ()

Interact with stdin and stdout by using a function to transform input to output. May be lazy. See interact for more.

readFile :: FilePath -> IO full

Read file. May be lazy.

writeFile :: FilePath -> full -> IO ()

Write data to file.

appendFile :: FilePath -> full -> IO ()

Append data to file.

Instances

ListLikeIO String Char

ListLikeIO ByteString Word8

ListLikeIO ByteString Word8

ListLikeIO Text Char

ListLikeIO Text Char

ListLikeIO CharStringLazy Char

ListLikeIO CharString Char

ListLikeIO (Seq Char) Char

(Integral i, Ix i) => ListLikeIO (Array i Char) Char

Special lists

Strings

"Set" operations

Ordered lists

Generalized functions

The "By" operations

User-supplied equality (replacing an Eq context)

User-supplied comparison (replacing an Ord context)

The "generic" operations

Notes on specific instances

Lists

Functions for operating on regular lists almost all use the native implementations in Data.List, Prelude, or similar standard modules. The exceptions are:

• mapM uses the default ListLike implementation
• hGet does not exist for String in the Haskell modules. It is implemented in terms of Data.ByteString.Lazy.
• hGetNonBlocking is the same way.

Arrays

Array is an instance of ListLike. Here are some notes about it:

• The index you use must be an integral
• ListLike functions that take an index always take a 0-based index for compatibility with other ListLike instances. This is translated by the instance functions into the proper offset from the bounds in the Array.
• ListLike functions preserve the original Array index numbers when possible. Functions such as cons will reduce the lower bound to do their job. snoc and append increase the upper bound. drop raises the lower bound and take lowers the upper bound.
• Functions that change the length of the array by an amount not known in advance, such as filter, will generate a new array with the lower bound set to 0. Furthermore, these functions cannot operate on infinite lists because they must know their length in order to generate the array. hGetContents and its friends will therefore require the entire file to be read into memory before processing is possible.
• empty, singleton, and fromList also generate an array with the lower bound set to 0.
• Many of these functions will generate runtime exceptions if you have not assigned a value to every slot in the array.

ByteStrings

Both strict and lazy ByteStreams can be used with ListLike.

ByteString ListLike instances operate on Word8 elements. This is because both Data.ByteString.ByteString and Data.ByteString.Char8.ByteString have the same underlying type. If you wish to use the Char8 representation, the newtype wrappers CharString and CharStringLazy are available.

Most ListLike operations map directly to ByteStream options. Notable exceptions:

• map uses the ListLike implementation. rigidMap is more efficient. The same goes for concatMap vs. rigidConcatMap.
• isInfixOf, sequence, mapM and similar monad operations, insert, union, intersect, sortBy, and similar functions are not implemented in ByteStream and use a naive default implementation.
• The lazy ByteStream module implements fewer funtions than the strict ByteStream module. In some cases, default implementations are used. In others, notably related to I/O, the lazy ByteStreams are converted back and forth to strict ones as appropriate.

newtype CharString

Newtype wrapper around Data.ByteString.Char8.ByteString, this allows for ListLike instances with Char elements.

Constructors

CS

Fields

unCS :: ByteString

Instances

Eq CharString

Ord CharString

Read CharString

Show CharString

Monoid CharString

StringLike CharString

FoldableLL CharString Char

ListLike CharString Char

ListLikeIO CharString Char

newtype CharStringLazy

Newtype wrapper around Data.ByteString.Lazy.Char8.ByteString, this allows for ListLike instances with Char elements.

Constructors

CSL

Fields

unCSL :: ByteString

Instances

Eq CharStringLazy

Ord CharStringLazy

Read CharStringLazy

Show CharStringLazy

Monoid CharStringLazy

StringLike CharStringLazy

FoldableLL CharStringLazy Char

ListLike CharStringLazy Char

ListLikeIO CharStringLazy Char

Base Typeclasses

The ListLike class

class (FoldableLL full item, Monoid full) => ListLike full item | full -> item where

The class implementing list-like functions.

It is worth noting that types such as Map can be instances of ListLike. Due to their specific ways of operating, they may not behave in the expected way in some cases. For instance, cons may not increase the size of a map if the key you have given is already in the map; it will just replace the value already there.

Implementators must define at least:

• singleton
• head
• tail
• null or genericLength

Methods

empty :: full

The empty list

singleton :: item -> full

Creates a single-element list out of an element

cons :: item -> full -> full

Like (:) for lists: adds an element to the beginning of a list

snoc :: full -> item -> full

Adds an element to the *end* of a ListLike.

append :: full -> full -> full

Combines two lists. Like (++).

head :: full -> item

Extracts the first element of a ListLike.

last :: full -> item

Extracts the last element of a ListLike.

tail :: full -> full

Gives all elements after the head.

init :: full -> full

All elements of the list except the last one. See also inits.

null :: full -> Bool

Tests whether the list is empty.

length :: full -> Int

Length of the list. See also genericLength.

map :: ListLike full' item' => (item -> item') -> full -> full'

Apply a function to each element, returning any other valid ListLike. rigidMap will always be at least as fast, if not faster, than this function and is recommended if it will work for your purposes. See also mapM.

rigidMap :: (item -> item) -> full -> full

Like map, but without the possibility of changing the type of the item. This can have performance benefits for things such as ByteStrings, since it will let the ByteString use its native low-level map implementation.

reverse :: full -> full

Reverse the elements in a list.

intersperse :: item -> full -> full

Add an item between each element in the structure

concat :: (ListLike full' full, Monoid full) => full' -> full

Flatten the structure.

concatMap :: ListLike full' item' => (item -> full') -> full -> full'

Map a function over the items and concatenate the results. See also rigidConcatMap.

rigidConcatMap :: (item -> full) -> full -> full

Like concatMap, but without the possibility of changing the type of the item. This can have performance benefits for some things such as ByteString.

any :: (item -> Bool) -> full -> Bool

True if any items satisfy the function

all :: (item -> Bool) -> full -> Bool

True if all items satisfy the function

maximum :: Ord item => full -> item

The maximum value of the list

minimum :: Ord item => full -> item

The minimum value of the list

replicate :: Int -> item -> full

Generate a structure with the specified length with every element set to the item passed in. See also genericReplicate

take :: Int -> full -> full

Takes the first n elements of the list. See also genericTake.

drop :: Int -> full -> full

Drops the first n elements of the list. See also genericDrop

splitAt :: Int -> full -> (full, full)

Equivalent to (take n xs, drop n xs). See also genericSplitAt.

takeWhile :: (item -> Bool) -> full -> full

Returns all elements at start of list that satisfy the function.

dropWhile :: (item -> Bool) -> full -> full

Drops all elements form the start of the list that satisfy the function.

span :: (item -> Bool) -> full -> (full, full)

The equivalent of (takeWhile f xs, dropWhile f xs)

break :: (item -> Bool) -> full -> (full, full)

The equivalent of span (not . f)

group :: (ListLike full' full, Eq item) => full -> full'

Split a list into sublists, each which contains equal arguments. For order-preserving types, concatenating these sublists will produce the original list. See also groupBy.

inits :: ListLike full' full => full -> full'

All initial segments of the list, shortest first

tails :: ListLike full' full => full -> full'

All final segnemts, longest first

isPrefixOf :: Eq item => full -> full -> Bool

True when the first list is at the beginning of the second.

isSuffixOf :: Eq item => full -> full -> Bool

True when the first list is at the beginning of the second.

isInfixOf :: Eq item => full -> full -> Bool

True when the first list is wholly containted within the second

elem :: Eq item => item -> full -> Bool

True if the item occurs in the list

notElem :: Eq item => item -> full -> Bool

True if the item does not occur in the list

find :: (item -> Bool) -> full -> Maybe item

Take a function and return the first matching element, or Nothing if there is no such element.

filter :: (item -> Bool) -> full -> full

Returns only the elements that satisfy the function.

partition :: (item -> Bool) -> full -> (full, full)

Returns the lists that do and do not satisfy the function. Same as (filter p xs, filter (not . p) xs)

index :: full -> Int -> item

The element at 0-based index i. Raises an exception if i is out of bounds. Like (!!) for lists.

elemIndex :: Eq item => item -> full -> Maybe Int

Returns the index of the element, if it exists.

elemIndices :: (Eq item, ListLike result Int) => item -> full -> result

Returns the indices of the matching elements. See also findIndices

findIndex :: (item -> Bool) -> full -> Maybe Int

Take a function and return the index of the first matching element, or Nothing if no element matches

findIndices :: ListLike result Int => (item -> Bool) -> full -> result

Returns the indices of all elements satisfying the function

sequence :: (Monad m, ListLike fullinp (m item)) => fullinp -> m full

Evaluate each action in the sequence and collect the results

mapM :: (Monad m, ListLike full' item') => (item -> m item') -> full -> m full'

A map in monad space. Same as sequence . map

See also rigidMapM

rigidMapM :: Monad m => (item -> m item) -> full -> m full

Like mapM, but without the possibility of changing the type of the item. This can have performance benefits with some types.

nub :: Eq item => full -> full

Removes duplicate elements from the list. See also nubBy

delete :: Eq item => item -> full -> full

Removes the first instance of the element from the list. See also deleteBy

deleteFirsts :: Eq item => full -> full -> full

List difference. Removes from the first list the first instance of each element of the second list. See '(\)' and deleteFirstsBy

union :: Eq item => full -> full -> full

List union: the set of elements that occur in either list. Duplicate elements in the first list will remain duplicate. See also unionBy.

intersect :: Eq item => full -> full -> full

List intersection: the set of elements that occur in both lists. See also intersectBy

sort :: Ord item => full -> full

Sorts the list. On data types that do not preserve ordering, or enforce their own ordering, the result may not be what you expect. See also sortBy.

insert :: Ord item => item -> full -> full

Inserts the element at the last place where it is still less than or equal to the next element. On data types that do not preserve ordering, or enforce their own ordering, the result may not be what you expect. On types such as maps, this may result in changing an existing item. See also insertBy.

toList :: full -> [item]

Converts the structure to a list. This is logically equivolent to fromListLike, but may have a more optimized implementation.

fromList :: [item] -> full

Generates the structure from a list.

fromListLike :: ListLike full' item => full -> full'

Converts one ListLike to another. See also toList. Default implementation is fromListLike = map id

nubBy :: (item -> item -> Bool) -> full -> full

Generic version of nub

deleteBy :: (item -> item -> Bool) -> item -> full -> full

Generic version of deleteBy

deleteFirstsBy :: (item -> item -> Bool) -> full -> full -> full

Generic version of deleteFirsts

unionBy :: (item -> item -> Bool) -> full -> full -> full

Generic version of union

intersectBy :: (item -> item -> Bool) -> full -> full -> full

Generic version of intersect

groupBy :: (ListLike full' full, Eq item) => (item -> item -> Bool) -> full -> full'

Generic version of group.

sortBy :: (item -> item -> Ordering) -> full -> full

Sort function taking a custom comparison function

insertBy :: (item -> item -> Ordering) -> item -> full -> full

Like insert, but with a custom comparison function

genericLength :: Num a => full -> a

Length of the list

genericTake :: Integral a => a -> full -> full

Generic version of take

genericDrop :: Integral a => a -> full -> full

Generic version of drop

genericSplitAt :: Integral a => a -> full -> (full, full)

Generic version of splitAt

genericReplicate :: Integral a => a -> item -> full

Generic version of replicate

Instances

ListLike ByteString Word8

ListLike ByteString Word8

ListLike Text Char

ListLike Text Char

ListLike CharStringLazy Char

ListLike CharString Char

ListLike [a] a

(Monoid (v a), Eq (v a), Vector v a) => ListLike (v a) a

ListLike (Seq a) a

ListLike (Vector a) a

Unbox a => ListLike (Vector a) a

Storable a => ListLike (Vector a) a

(Integral i, Ix i) => ListLike (Array i e) e

The FoldableLL class

class FoldableLL full item | full -> item where

This is the primary class for structures that are to be considered foldable. A minimum complete definition provides foldl and foldr.

Instances of FoldableLL can be folded, and can be many and varied.

These functions are used heavily in Data.ListLike.

Methods

foldl :: (a -> item -> a) -> a -> full -> a

Left-associative fold

foldl' :: (a -> item -> a) -> a -> full -> a

Strict version of foldl.

foldl1 :: (item -> item -> item) -> full -> item

A variant of foldl with no base case. Requires at least 1 list element.

foldr :: (item -> b -> b) -> b -> full -> b

Right-associative fold

foldr' :: (item -> b -> b) -> b -> full -> b

Strict version of foldr

foldr1 :: (item -> item -> item) -> full -> item

Like foldr, but with no starting value

Instances

FoldableLL ByteString Word8

FoldableLL ByteString Word8

FoldableLL Text Char

FoldableLL Text Char

FoldableLL CharStringLazy Char

FoldableLL CharString Char

FoldableLL [a] a

Vector v a => FoldableLL (v a) a

FoldableLL (Seq a) a

FoldableLL (Vector a) a

Unbox a => FoldableLL (Vector a) a

Storable a => FoldableLL (Vector a) a

Ix i => FoldableLL (Array i e) e

The StringLike class

class StringLike s where

An extension to ListLike for those data types that are similar to a String. Minimal complete definition is toString and fromString.

Methods

toString :: s -> String

Converts the structure to a String

fromString :: String -> s

Converts a String to a list

lines :: ListLike full s => s -> full

Breaks a string into a list of strings

words :: ListLike full s => s -> full

Breaks a string into a list of words

Instances

StringLike String

StringLike ByteString

StringLike ByteString

StringLike Text

StringLike Text

StringLike CharStringLazy

StringLike CharString

(Eq (v Char), Vector v Char) => StringLike (v Char)

StringLike (Seq Char)

StringLike (Vector Char)

StringLike (Vector Char)

StringLike (Vector Char)

(Integral i, Ix i) => StringLike (Array i Char)

The InfiniteListLike class

class ListLike full item => InfiniteListLike full item | full -> item where

An extension to ListLike for those data types that are capable of dealing with infinite lists. Some ListLike functions are capable of working with finite or infinite lists. The functions here require infinite list capability in order to work at all.

Methods

iterate :: (item -> item) -> item -> full

An infinite list of repeated calls of the function to args

repeat :: item -> full

An infinite list where each element is the same

cycle :: full -> full

Converts a finite list into a circular one

Instances

InfiniteListLike [a] a