@hackage mini-egison0.1.0

This package provides the pattern-matching facility that fulfills the following three criteria for practical pattern matching for non-free data types: (i) non-linear pattern matching with backtracking; (ii) extensibility of pattern-matching algorithms; (iii) ad-hoc polymorphism of patterns. Non-free data types are data types whose data have no standard forms. For example, multisets are non-free data types because the multiset '[a,b,b]' has two other equivalent but literally different forms '[b,a,b]' and '[b,b,a]'.

The design of the pattern-matching facility is originally proposed in this paper and implemented in the Egison programming language.

Samples

We can extract all twin primes from the list of prime numbers by pattern matching:

take 10 (matchAll primes (List Integer)
           [[mc| join _ (cons $p (cons #(p+2) _)) => (p, p+2) |]])
-- [(3,5),(5,7),(11,13),(17,19),(29,31),(41,43),(59,61),(71,73),(101,103),(107,109)]

We can describe patterns for each poker hand utilizing pattern matching for a multiset:

poker cs =
  match cs (Multiset CardM)
    [[mc| cons (card $s $n)
           (cons (card #s #(n-1))
            (cons (card #s #(n-2))
             (cons (card #s #(n-3))
              (cons (card #s #(n-4))
               _)))) => "Straight flush" |],
     [mc| cons (card _ $n)
           (cons (card _ #n)
            (cons (card _ #n)
             (cons (card _ #n)
              (cons _
               _)))) => "Four of a kind" |],
     [mc| cons (card _ $m)
           (cons (card _ #m)
            (cons (card _ #m)
             (cons (card _ $n)
              (cons (card _ #n)
               _)))) => "Full house" |],
     [mc| cons (card $s _)
           (cons (card #s _)
            (cons (card #s _)
             (cons (card #s _)
              (cons (card #s _)
               _)))) => "Flush" |],
     [mc| cons (card _ $n)
           (cons (card _ #(n-1))
            (cons (card _ #(n-2))
             (cons (card _ #(n-3))
              (cons (card _ #(n-4))
               _)))) => "Straight" |],
     [mc| cons (card _ $n)
           (cons (card _ #n)
            (cons (card _ #n)
             (cons _
              (cons _
               _)))) => "Three of a kind" |],
     [mc| cons (card _ $m)
           (cons (card _ #m)
            (cons (card _ $n)
             (cons (card _ #n)
              (cons _
               _)))) => "Two pair" |],
     [mc| cons (card _ $n)
           (cons (card _ #n)
            (cons _
             (cons _
              (cons _
               _)))) => "One pair" |],
     [mc| _ => "Nothing" |]]

The pattern-matching algorithms for List and Multiset can be defined by users.