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Manual Reference Pages  -  LIST::GEN::HASKELL (3)

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NAME

List::Gen::Haskell - the haskell prelude in perl5

CONTENTS

SYNOPSIS

this module provides most of the functions in the haskell prelude that pertain to working with lists.



    # haskell:  fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

    use List::Gen::Haskell;

    $fibs = lazy 0, 1, zipWith {&sum} $fibs, tail $fibs;

    print "@$fibs[0 .. 10]\n"; # prints 0 1 1 2 3 5 8 13 21 34 55



lazy provides the generic behavior of haskell’s lazy list concatenation (the : and ++ operators). none of its elements are touched until they are needed. in general, all the functions in this package defer their execution until something is required of them. they also only touch their arguments at the latest possible time, which is how the co-recursion above works.

the functions in this module are a bit more flexible (perlish) than those in haskell. in most cases where a function expects a single generator, a list of values and/or generators can be provided, which will be preprocessed by lazy into a single generator.

when loaded, most of the functions in this package become methods for all generators. if a method of the same name already exists, the method from this package will be prefixed with hs_ . so filter is a method named ->hs_filter(...)

this library currently does not have the best performance due to over-caching of many internal generators. a future update will address this by replacing those generators with cache-less generator streams.

FUNCTIONS

all of these functions are available with a ucfirst name, since many clash with perl builtin names.

    Utility Operations

x_xs [GENERATOR] x_xs is a convenience function that returns the head and tail of a passed in generator. x_xs uses $_ without an argument.
seq LIST forces immediate evaluation of the elements in LIST and returns the list
flip {CODE} flip converts CODE into a function that takes it’s arguments reversed



    my $idx = \&head . flip \&drop;

    $idx->($gen, 5)  ==  $gen->$idx(5)  ==  $gen->get(5)



    List operations

Map {CODE} LIST Map f xs is the list obtained by applying f to each element of xs, i.e.,



    map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
    map f [x1, x2, ...] == [f x1, f x2, ...]

    $x = &map(sub {$_**2}, $gen);
    $x = Map {$_**2} $gen;



that usage is the same as List::Gen::gen , but this is something that gen can’t do:



    my $pow_2 = Map {$_**2};  # partial application, needs at least 1 more
                              # argument to evaluate, but can be passed a list
    my $ints = <0..>;

    my $squares = $ints->$pow_2;

    say "@$squares[0 .. 10]"; # 0 1 4 9 16 25 36 49 64 81 100



and this:



    my $src;
    my $square_of_src = Map {$_ ** 2} $src;

    $src = <1.. by 2>;

    say "@$square_of_src[0 .. 4]"; # 1 9 25 49 81



filter {CODE} LIST filter, applied to a predicate and a list, returns the list of those elements that satisfy the predicate; i.e.,



    filter p xs = [ x | x <- xs, p x]



head GENERATOR Extract the first element of a list, which must be non-empty.
Last GENERATOR Extract the last element of a list, which must be finite and non-empty.
tail GENERATOR Extract the elements after the head of a list, which must be non-empty.
init GENERATOR Return all the elements of a list except the last one. The list must be non-empty.
null GENERATOR Test whether a list is empty.
Length GENERATOR
Reverse LIST reverse xs returns the elements of xs in reverse order. xs must be finite.

    Reducing lists (folds)

foldl {CODE} ITEM LIST foldl, applied to a binary operator, a starting value (typically the left-identity of the operator), and a list, reduces the list using the binary operator, from left to right:



    foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn



The list must be finite.

foldl1 {CODE} LIST foldl1 is a variant of foldl that has no starting value argument, and thus must be applied to non-empty lists.
foldr {CODE} ITEM LIST foldr, applied to a binary operator, a starting value (typically the right-identity of the operator), and a list, reduces the list using the binary operator, from right to left:



    foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)



foldr1 {CODE} LIST foldr1 is a variant of foldr that has no starting value argument, and thus must be applied to non-empty lists.
Special folds
And LIST and returns the conjunction of a Boolean list. For the result to be True, the list must be finite; False, however, results from a False value at a finite index of a finite or infinite list.
Or LIST or returns the disjunction of a Boolean list. For the result to be False, the list must be finite; True, however, results from a True value at a finite index of a finite or infinite list.
any {CODE} LIST Applied to a predicate and a list, any determines if any element of the list satisfies the predicate.
all {CODE} LIST Applied to a predicate and a list, all determines if all elements of the list satisfy the predicate.
sum LIST The sum function computes the sum of a finite list of numbers.
product LIST The product function computes the product of a finite list of numbers.
concat GENERATOR Concatenate a list of lists.
concatMap {CODE} LIST Map a function over a list and concatenate the results.
maximum LIST maximum returns the maximum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of maximumBy, which allows the programmer to supply their own comparison function.
minimum LIST minimum returns the minimum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of minimumBy, which allows the programmer to supply their own comparison function.

    Building lists

Scans
scanl {CODE} LIST scanl is similar to foldl, but returns a list of successive reduced values from the left:



    scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]



Note that



    last (scanl f z xs) == foldl f z xs.



scanr {CODE} LIST scanr is the right-to-left dual of scanl. Note that



    head (scanr f z xs) == foldr f z xs.



Infinite lists
iterate {CODE} ITEM iterate f x returns an infinite list of repeated applications of f to x:



    iterate f x == [x, f x, f (f x), ...]



repeat ITEM repeat x is an infinite list, with x the value of every element.
hs_repeat ITEM


    my $repeat; $repeat = lazy $x, $repeat;



replicate NUM ITEM replicate n x is a list of length n with x the value of every element.
cycle LIST cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.
hs_cycle LIST hs_cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.

it is defined in perl as:



   my $cycle; $cycle = lazy $xs, $cycle;



Sublists
take NUM LIST take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n > length xs:



    take 3 [1,2,3,4,5] == [1,2,3]
    take 3 [1,2] == [1,2]
    take 3 [] == []
    take (-1) [1,2] == []
    take 0 [1,2] == []



drop NUM LIST drop n xs returns the suffix of xs after the first n elements, or [] if n > length xs:



    drop 3 [1,2,3,4,5] == [4,5]
    drop 3 [1,2] == []
    drop 3 [] == []
    drop (-1) [1,2] == [1,2]
    drop 0 [1,2] == [1,2]



splitAt NUM LIST splitAt n xs returns a tuple where first element is xs prefix of length n and second element is the remainder of the list:



    splitAt 3 [1,2,3,4,5] == ([1,2,3],[4,5])
    splitAt 1 [1,2,3] == ([1],[2,3])
    splitAt 3 [1,2,3] == ([1,2,3],[])
    splitAt 4 [1,2,3] == ([1,2,3],[])
    splitAt 0 [1,2,3] == ([],[1,2,3])
    splitAt (-1) [1,2,3] == ([],[1,2,3])



It is equivalent to (take n xs, drop n xs).

takeWhile {CODE} LIST takeWhile, applied to a predicate p and a list xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p:



    take_while (< 3) [1,2,3,4,1,2,3,4] == [1,2]
    take_while (< 9) [1,2,3] == [1,2,3]
    take_while (< 0) [1,2,3] == []



dropWhile {CODE} LIST dropWhile p xs returns the suffix remaining after take_while p xs:



    dropWhile (< 3) [1,2,3,4,5,1,2,3] == [3,4,5,1,2,3]
    dropWhile (< 9) [1,2,3] == []
    dropWhile (< 0) [1,2,3] == [1,2,3]



span {CODE} LIST span, applied to a predicate p and a list xs, returns a tuple where first element is longest prefix (possibly empty) of xs of elements that satisfy p and second element is the remainder of the list:



    span (< 3) [1,2,3,4,1,2,3,4] == ([1,2],[3,4,1,2,3,4])
    span (< 9) [1,2,3] == ([1,2,3],[])
    span (< 0) [1,2,3] == ([],[1,2,3])



span p xs is equivalent to (takeWhile p xs, dropWhile p xs)

break {CODE} LIST break, applied to a predicate p and a list xs, returns a tuple where first element is longest prefix (possibly empty) of xs of elements that do not satisfy p and second element is the remainder of the list:



    break (> 3) [1,2,3,4,1,2,3,4] == ([1,2,3],[4,1,2,3,4])
    break (< 9) [1,2,3] == ([],[1,2,3])
    break (> 9) [1,2,3] == ([1,2,3],[])



break p is equivalent to span (not . p)

Searching lists
elem ITEM LIST elem is the list membership predicate, usually written in infix form, e.g., x ‘elem‘ xs.
notElem ITEM LIST notElem is the negation of elem.
Zipping and unzipping lists
zip LIST zip takes 2+ lists and returns a single interleaved list. If one input list is short, excess elements of the longer lists are discarded. unlike the haskell version, the zip returns a flat generator.

zip is the same as zipWith {\@_}

zipWith {CODE} LIST zipWith generalizes zip by zipping with the function given as the first argument, instead of a tupling function. For example, zipWith (+) is applied to two lists to produce the list of corresponding sums.
zipWithAB {$a * $b} $gen1, $gen2 The zipWithAB function takes a function which uses $a and $b , as well as two lists and returns a list analogous to zipWith.
unzip GENERATOR unzip transforms a list into two lists of the even and odd elements.



    zs = zip xs, ys
    (xs, ys) == unzip zs



unzipn NUM GENERATOR The unzipn function is the n-dimentional precursor to unzip



   unzip xs = unzipn 2, xs



Functions on strings
lines STRING lines breaks a string up into a list of strings at newline characters. The resulting strings do not contain newlines. the newline sequence is taken from the value of the input record separator $/
words STRING words breaks a string up into a list of words, which were delimited by white space.
unlines LIST unlines is an inverse operation to lines. It joins lines, after appending a terminating newline to each. the newline sequence is taken from the value of the input record separator $/
unwords LIST unwords is an inverse operation to words. It joins words with separating spaces.

ACKNOWLEGEMENTS

most of the documentation here started out at <http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Prelude.html> and was subsequently edited to account for implementation differences.

AUTHOR

Eric Strom, <asg at cpan.org>

BUGS

there are certainly bugs in code this complex. send in reports, tests, patches.

report any bugs / feature requests to bug-list-gen at rt.cpan.org, or through the web interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=List-Gen>.

COPYRIGHT & LICENSE

copyright 2009-2011 Eric Strom.

this program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

see http://dev.perl.org/licenses/ for more information.

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perl v5.20.3 LIST::GEN::HASKELL (3) 2011-11-12

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