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More Algorithms and Data Structures, using Haskell


General-purpose algorithms and data structures, illustrated in Haskell. Part II


Pure functional, mutation-free, efficient double-linked lists

It is always an interesting challenge to write a pure functional and efficient implementation of an imperative algorithm destructively operating on a data structure. The functional implementation has a significant benefit of equational reasoning and modularity. We can comprehend the algorithm without keeping the implicit global state in mind. The mutation-free, functional realization also has practical benefits: the ease of adding checkpointing, undo and redo. The absence of mutations makes the code multi-threading-safe and helps in porting to distributed or non-shared-memory parallel architectures. On the other hand, an imperative implementation has the advantage of optimality: mutating a component in a complex data structure is a constant-time operation, at least on conventional architectures. Imperative code makes sharing explicit, and so permits efficient implementation of cyclic data structures.

We show a simple example of achieving all the benefits of an imperative data structure -- including sharing and the efficiency of updates -- in a pure functional program. Our data structure is a doubly-linked, possibly cyclic list, with the standard operations of adding, deleting and updating elements; traversing the list in both directions; iterating over the list, with cycle detection. The code:

The algorithm is essentially imperative -- hence supporting the identity check and the in-place `update' -- but implemented purely functionally. Although the code relies on many local, type safe `heaps', there is emphatically no global heap and no global state.

It is not for nothing that Haskell has been called the best imperative language. Imperative algorithms are implementable as they are -- yet genuinely functionally, without resorting to the monadic sub-language but taking the full advantage of clausal definitions, pattern guards, laziness.

The current version is 1.2, Jan 7, 2009.
FDList.hs [8K]
The complete, commented Haskell98 code and many tests

Haskell-Cafe discussion ``Updating doubly linked lists''. January 2009.


Total stream processors and their applications to all infinite streams

In the article on seemingly impossible functional programs, Marti'n Escardo' wrote about decidable checking of the satisfaction of a total computable predicate on Cantor numerals. The latter represent infinite bit strings, or all real numbers within [0,1]. Mart'n Escardo's technique can tell, in finite time, if a given total computable predicate is satisfied over all possible infinite bit strings. Furthermore, for so-called sparse predicates, Marti'n Escardo's technique is very fast.

We re-formulate the problem in terms of streams and depth-limited depth-first search, and thus cast off the mystery of deciding the satisfiability of a total computable predicate over the set of all Cantor numerals, which are uncountable.

As an additional contribution, we show how to write functions over Cantor numerals in a `natural' monadic style so that those functions become self-partially evaluating. The instantiation of the functions in an appropriate pure monad gives us transparent memoization, without any changes to the functions themselves. The monad in question is pure and involves no reference cells.

On `dense' functions on numerals (i.e., those that need to examine most of the bits of their argument, up to a limit), our technique performs about 9 times faster than the most sophisticated one by Marti'n Escardo'.

The current version is October 2, 2007.
Total stream processors and quantification over the infinite number of infinite streams. The complete article.
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StreamPEval.hs [12K]
Extensively commented Haskell98 code

Marti'n Escardo': Seemingly impossible functional programs.
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Generating interesting lambda-terms

It is straightforward to generate lambda-terms, with the help of the simplest non-deterministic operations for choice and failure that are available in many language systems. With a little more care one can ensure that any lambda-term occur in the generated stream sooner or later. It takes remarkably more care to make interesting terms come sooner than much, much later. This article gives a few tips on how to relatively quickly generate interesting terms such the S and Y combinators, the divergent omega-term, a Church numeral and its successor.

We limit attention to closed terms since all interesting terms mentioned earlier are closed. De Bruijn indices let us avoid wasting time on building alpha-equivalent terms. (The terms will be pretty-printed in the conventional, named-variable notation, for clarity.) Thus we will be generating terms described by the following grammar (data type declaration), with the side condition that the terms are closed.

     data Exp = V Int | A Exp Exp | L Exp

The generator is a truly straightforward application of the simplest non-deterministic operations in the MonadPlus interface.

     a_term_naive :: MonadPlus m => m Exp
     a_term_naive = L `liftM` go 0
      -- go n: generate a lambda term with free variables 0..n
      go n = choose_var n `mplus` choose_lam n `mplus` choose_app n
      choose_var 0 = return (V 0)
      choose_var n = return (V n) `mplus` choose_var (n-1)  
      choose_app n = liftM2 A (go n) (go n)
      choose_lam n = L `liftM` go (n+1)

Absent constants, a closed lambda-term must start with a lambda-binder. The rest could be either a variable, an application of two non-deterministically chosen terms, or an abstraction with a random body.

The equally straightforward implementation of MonadPlus, the List monad, fails to generate anything interesting within the first 10 000 terms. It is not difficult to see why. Here are the first 8 generated terms:

     Lx.x,  Lx.Ly.x,  Lx.Ly.y,  Lx.Ly.Lz.x,  Lx.Ly.Lz.y,  Lx.Ly.Lz.z,  Lx.Ly.Lz.Lu.x,  Lx.Ly.Lz.Lu.y

The List monad realizes an incomplete search strategy: there is no guarantee that any given term will ever come. No applications are indeed forthcoming with the List monad. A better implementation of non-determinism is needed, with a complete search strategy: for example, iterative deepening or FBackTrack. With iterative deepening (which in our implementation produces the same sequence as breadth-first search but without taking Gbytes of memory), the first 10 generated terms

     Lx.x,  Lx.Ly.x,  Lx.Ly.y,  Lx.x x,  Lx.Ly.Lz.x,  Lx.Ly.Lz.y,  Lx.Ly.Lz.z, Lx.x (Ly.x), Lx.x (Ly.y), Lx.x (x x)

look quite hopeful. Within the first 10 000 generated terms, the self-application Lx.x x, or delta, comes 4th; the third Church numeral comes 716th and omega comes 3344th. Alas, there is no successor or the S combinator, let alone the Y combinator. With the FBackTrack implementation, the first few terms may look even more hopeful

     Lx.x,  Lx.x x,  Lx.Ly.x,  Lx.x x x,  Lx.Ly.x x,  Lx.x (x x),  Lx.Ly.Lz.x,  Lx.(Ly.x) x,  Lx.x (Ly.x)
The term delta comes the second, the third Church numeral comes 695th. Alas, within the first 99 977 terms nothing else interesting comes. Generating interesting terms is not at all as straightforward as it seems. Even sophisticated MonadPlus implementations did not help.

It is crucial to recognize that the straightforward search for lambda-terms is biased, and that bias is not in favor of interesting terms. Generally, the fewer non-deterministic choices it takes to produce a term, the sooner the term comes. The straightforward generator clearly favors selectors (terms of the form Lx.Ly...Lu.x) and simple applications such as Lx. (x x) (x x). We should put the brake on generating abstractions: each new L adds a new variable and hence dramatically many more possible terms. We should encourage the generator to play more with the variables it already has. To prevent the string of applications of a variable to itself, we should produce terms in the general order of their size. Again the goal is to explore the search space more uniformly. It is tempting to generate only normal forms. Alas, the Y combinator and omega do not have normal forms. Therefore, we do generate redices, but only of interesting kinds (whose argument is an abstraction), and only occasionally. The following code incorporates these ideas:

     a_term :: MonadPlus m => m Exp
     a_term = L `liftM` go 0
      -- go n: generate a lambda term with free variables 0..n
      go n = do
        size <- iota 1
        gen n True size
      -- gen n l s: generate a lambda term with free variables 0..n and
      -- of the size exactly s. The second argument tells if to generate
      -- abstractions
      gen _ _ s | s <= 0 = mzero
      gen n _     1 = choose_var n
      gen n True  2 = choose_lam n 2
      gen n False 2 = mzero
      gen n True  s = choose_app n s `mplus` choose_lam n s
      gen n False s = choose_app n s
      choose_var n = msum . map (return . V) $ [0..n]
      choose_lam n s = penalty (40*n) $ L `liftM` gen (n+1) True (s-1)
      choose_app n s = do
        let s' = s - 1                       -- Account for the 'A' constructor
        let gen_redex = do
                        lefts <- range 4 (s' - 3)
                        liftM2 A (choose_lam n lefts) (choose_lam n (s' - lefts))
        let gen_noredex = do
                          lefts <- range 1 (s' - 1)
                          liftM2 A (gen n False lefts) (gen n True (s'-lefts))
        gen_noredex `mplus` penalty 4 gen_redex

The auxiliary iota i produces an integer greater or equal to i; range i j chooses an integer between i and j, inclusive. The operation yield

     -- Lowers the priority of m, so choices of m will be tried less often
     yield :: MonadPlus m => m a -> m a
     yield m = mzero `mplus` m
and its n-th iterate penalty n produce a string of failures before trying the argument computation m. When a complete search strategy sees many failures it tends to turn away and to pay more attention to other parts of the search space.

Here are the first 10 terms produced by the sophisticated generator:

     Lx.x,  Lx.Ly.y,  Lx.Ly.x,  Lx.x x,  Lx.x (Ly.y),  Lx.Ly.y y,  Lx.Ly.x y,  Lx.x (x x),  Lx.Ly.y x,  Lx.x (Ly.x)
The term delta comes fourth, omega comes 54th, the Y combinator 303d, the third Church numeral 393d, the S combinator 1763d and the successor 4865th.

The conclusion, although obvious in hindsight, is still thought-provoking: interesting lambda-terms are really hard to encounter by accident. They are exquisitely rare in the space of possible lambda-terms and distributed non-uniformly. A monkey banging on even the sophisticated lambda-typewriter may have printed the Y combinator, but would unlikely to print even the addition combinator within its lifetime.

EnumFix.hs [12K]
The complete code for the simple and sophisticated generators, as part of the procedure to search for fixpoint combinators

FBackTrack.hs [3K]
Simple fair and terminating backtracking monad

Searches.hs [9K]
Iterative deepening search


Efficient integer logarithm of large numbers in any base

The integer logarithm of the number n in base b is the integer l such that b^l <= n < b^(l+1). The number one greater than the integer logarithm base b is the size of n, that is, the number of digits, in its base-b representation. We present the Haskell98 code that is just as efficient as the internal GHC.Integer.Logarithms.integerLogBase# function but uses no unboxed data, no optimizations, and is not even compiled.

Naively, the integer logarithm is computed by repeated divisions of n by b, until the result reaches 1. This procedure requires l divisions, where l is the logarithm. We present two efficient algorithms: the first uses only multiplications, no more than 2*log_2 l of them, and (log_2 l) * sizeof(n) extra memory for the powers of b. The second algorithm does log_2 l multiplications and no more than log_2 l integer divisions (and the same amount of extra memory) to compute l.

Here is the multiplication-only procedure that returns l+1 where l is the integer logarithm of n in base b. It is a composition of two functions, which together compute the bits of l: major_bit determines the upper bound and other_bits improves it.

     data BaseP = BaseP  !Integer -- b^k
                         !Int     -- k
     mul_bp :: BaseP -> BaseP -> BaseP
     mul_bp (BaseP bk1 k1) (BaseP bk2 k2) = BaseP (bk1*bk2) (k1+k2)
     basewidth :: Integer -> Integer -> Int
     basewidth b _ | b < 1 = error "basewidth: base must be greater than 1"
     basewidth b n | n < b = 1
     basewidth b n | n == b = 2
     basewidth b n = major_bit [BaseP b 1]
      major_bit :: [BaseP] -> Int
      major_bit bases@(bp:bps) =
        let bpnext@(BaseP bk2 k2) = bp `mul_bp` bp in
        case compare bk2 n of
          EQ -> k2 + 1                         -- n == b^(2k)
          GT -> other_bits bp bps              -- b^(2k) > n
          LT -> major_bit (bpnext : bases)     -- b^(2k) < n
      other_bits (BaseP _ i) [] = i+1          -- b^i < n < b^(i+1)
      other_bits bp (bphalf:bps) =
        let bpup@(BaseP bik ik) = bp `mul_bp` bphalf in
        case compare bik n of
          EQ -> ik + 1                         -- n == b^(i+k)
          GT -> other_bits bp bps              -- b^i < n < b^(i+k)
          LT -> other_bits bpup bps            -- b^(i+k) < n < b^(i+2k)
The correctness and the complexity analysis follow from the invariants of the two auxiliary functions. In any call major_bit bases, the list bases is [BaseP b^k k | j <- [d,d-1..0], k=2^j] for some d and n > b^(2^d). In the first invocation, d is 0 and progressively increases until such d>0 is found that b^(2^(d-1)) < n <= b^(2^d). In any call other_bits (BaseP bi i) bases, the list bases is [BaseP b^k k | j <- [d,d-1..0], k=2^j] for some d, k=2^d and b^i < n < b^(i+2k). Each invocation of other_bits halfs k until it reaches one. The other, more efficient as it turns out, algorithm modifies other_bits to divide n by the candidate lower bound b^i. That integer logarithm function computes that the 48th Mersenne prime 2^57885161-1 has 17425170 digits in its decimal representation -- in 1.2 seconds.
The current version is December 2013.
BaseWidth.hs [9K]
Complete commented Haskell98 source code and tests


Fast computation of Bernoulli numbers

This Haskell98 code quickly computes Bernoulli numbers. The code avoids explicit recursion, explicit factorials and (most) computing with rationals, demonstrating stream-wise processing and CAF memoization. For example, the following snippet defines a 2D table of pre-computed powers r^n for all r>=2 and n>1. Thanks to lazy evaluation, the table is automatically sized as needed. There is no need to guess the maximal size of the table so to allocate it.
     powers = [2..] : map (zipWith (*) (head powers)) powers
The rest of the algorithm exhibits similar stream-wise processing and computations of tables in terms of themselves.
The current version is 1.1, March 2003.
Bernoulli.hs [3K]
Commented Haskell98 source code and tests

Messages speedup help by Damien R. Sullivan, Bill Wood, Andrew J Bromage, Mark P Jones, and many others posted on the Haskell-Cafe mailing list on March 6-8, 2003
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Iterated zipping puzzles

The following is the refined code for the challenge originally posted by Joe English on a Haskell-Cafe thread about homework-like puzzles. The challenge was to figure out what the code does without first loading it up in a Haskell interpreter.
     s f g x = f x (g x)
     puzzle = (!!) $ iterate (s (lzw (+)) (0:)) [1] where
         lzw op [] ys = ys
         lzw op (x:xs) (y:ys) = op x y : lzw op xs ys

Incidentally, a small change gives a different series:

     puzzle1 = (!!) $ iterate (s ((lzw (+)).(0:)) (1:)) [] where
         lzw op [] ys = ys
         lzw op (x:xs) (y:ys) = op x y : lzw op xs ys

Finally, how can we possibly live without the following:

     puzzle2 = (!!) $ iterate (s ((lzw (+)).(1:).(0:)) (0:)) [1,1] where
         lzw op xs [] = []
         lzw op (x:xs) (y:ys) = op x y : lzw op xs ys


     *Main> puzzle 5
     *Main> puzzle1 5
     *Main> puzzle2 5
The current version is August 2003.
Message Homework posted on the Haskell-Cafe mailing list on Mon, 25 Aug 2003 18:50:42 -0700 (PDT)

Discussion thread, started on Haskell-Cafe by Thomas Bevan on Aug 22, 2003.
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