Std.Semigroup

Semigroup interface — types that support an associative combine.

A semigroup is any type a equipped with an associative binary operation. combine is the operation the concatenation operators desugar to: x <> y is combine(x, y), and x + y on a non-numeric operand is the same (a Swift-style + overload). Numeric + keeps its primitive meaning — only non-numeric operands route through here.

The built-in implementation for List is the ordinary structural append, a reducing library function (not a kernel primitive), so Vector/length- indexed reasoning keeps computing through a concatenation. String is nominal, so it supplies its own instance (Std.String.concat) rather than riding the List one.

Examples

use Std.Semigroup

combine([1, 2], [3, 4])        # => [1, 2, 3, 4]
[1, 2] <> [3, 4]               # => [1, 2, 3, 4]
"ab" <> "cd"                   # => "abcd"

Semigroup(a) equips a with an associative combine.