Algebraic Data Types for perl6, inspired by Haskell
Perl6
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README.md

Algebraic Data Types

This module implements algebraic data types inspired by the Haskell syntax. See below for limitations of the current implementation.

Here's a code example for you:

# define the algebraic data type
my $adt = q{Tree = Branch Tree left, Tree right | Leaf Str storage}
my %res = create_adt($adt);

# capture the Tree class
my \Tree = %res<Tree>;

# create a tree using named parameters
my $t =
    Tree.new-branch(
        :left(Tree.new-branch(
            :left(Tree.new-leaf(:storage(1))),
            :right(Tree.new-leaf(:storage(2))))),
        :right(Tree.new-leaf(:storage(3))));
# pretty-print the tree
say $t.gist;

# create a tree using positional arguments
my $t2 =
    Tree.new-branch(
        Tree.new-branch(
            Tree.new-leaf(1),
            Tree.new-leaf(2)),
        Tree.new-leaf(3));
say $t2.gist;

# capture the subtypes to pattern-match in a given block.
my \Branch = %res<Branch>;
my \Leaf = %res<Leaf>;

# map over a whole tree changing all storages.
sub treemap($t, *&code) {
    given $t {
        when Branch { return Tree.new-branch(treemap($t.left, &code), treemap($t.right, &code)) }
        when Leaf { return Tree.new-leaf(code($t.storage)) }
    }
}

# multiply every leaf node by 10
say treemap($t2, * * 10).gist;

Limitations

Compile-time availability

Since rakudo doesn't yet allow the EXPORT sub of a package to return a hash of things to export, the ADT cannot be used to define multi subs, because the symbols are not available at compile time.

When EXPORT works, code like this will be possible:

use ADT "data Tree = Branch Tree left, Tree right | Leaf Str storage";
multi sub treemap(Branch $t, *&c) { ... }
multi sub treemap(Leaf $l, *&c) { ... }

Compile-time checks

One of the neat features of ADT is that the compiler can ensure, that the program is prepared to deal with every defined case (Branch or Leaf, Just or Nothing, ...). A macro can probably be devised to implement such a compile-time check.

"is parsed" macros

When Rakudo gets support for "is parsed" macros, definitons of ADT can move from the "use" line into the mainline of the code, so that they look tidyer and the programmer can put comments nearby etc.