List::Gen::Haskell - the haskell prelude in perl5
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.
all of these functions are available with a ucfirst name, since many clash with perl builtin names.
[GENERATOR]
x_xs
is a convenience function that returns the head and tail of a passed in generator. x_xs
uses $_
without an argument.
LIST
forces immediate evaluation of the elements in LIST
and returns the list
{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)
{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
{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]
GENERATOR
Extract the first element of a list, which must be non-empty.
GENERATOR
Extract the last element of a list, which must be finite and non-empty.
GENERATOR
Extract the elements after the head of a list, which must be non-empty.
GENERATOR
Return all the elements of a list except the last one. The list must be non-empty.
GENERATOR
Test whether a list is empty.
GENERATOR
LIST
reverse xs returns the elements of xs in reverse order. xs must be finite.
{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.
{CODE} LIST
foldl1
is a variant of foldl
that has no starting value argument, and thus must be applied to non-empty lists.
{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)...)
{CODE} LIST
foldr1
is a variant of foldr
that has no starting value argument, and thus must be applied to non-empty lists.
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.
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.
{CODE} LIST
Applied to a predicate and a list, any determines if any element of the list satisfies the predicate.
{CODE} LIST
Applied to a predicate and a list, all determines if all elements of the list satisfy the predicate.
LIST
The sum function computes the sum of a finite list of numbers.
LIST
The product function computes the product of a finite list of numbers.
GENERATOR
Concatenate a list of lists.
{CODE} LIST
Map a function over a list and concatenate the results.
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.
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.
{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.
{CODE} LIST
scanr is the right-to-left dual of scanl. Note that
head (scanr f z xs) == foldr f z xs.
{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), ...]
ITEM
repeat x is an infinite list, with x the value of every element.
ITEM
my $repeat; $repeat = lazy $x, $repeat;
NUM ITEM
replicate n x is a list of length n with x the value of every element.
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.
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;
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] == []
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]
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).
{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] == []
{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]
{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)
{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)
ITEM LIST
elem is the list membership predicate, usually written in infix form, e.g., x `elem` xs.
ITEM LIST
notElem is the negation of elem.
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 {\@_}
{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.
{$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.
GENERATOR
unzip transforms a list into two lists of the even and odd elements.
zs = zip xs, ys (xs, ys) == unzip zs
NUM GENERATOR
The unzipn function is the n-dimentional precursor to unzip
unzip xs = unzipn 2, xs
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 $/
STRING
words breaks a string up into a list of words, which were delimited by white space.
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 $/
LIST
unwords is an inverse operation to words. It joins words with separating spaces.
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.
Eric Strom, <asg at cpan.org>
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 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.