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Comment by morsch | Hacker News Reader

The definition of a mold is clear (to anyone who has had higher education in mathematics, which seems a prerequisite for this introduction). What I don't have is any intuition about what a mold is.

The example helps a little. The tutorial seems to have another example of a mold in the final section, but I'm only guessing so because of that section's title. I'll be damned if I can tell where the mold is in that section and I cannot relate it at all to the example in your comment.

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From OP, in the section "Our first mold":

After seeing a few span examples, are we ready to describe the set of all spans with a Hoon mold? Well, no, but let's try it anyway. Ignore the syntax (which we'll explain later; this is a tutorial, not a reference manual), and you'll get the idea:

  ++  span
    $%  [%atom p=@tas]
        [%cell p=span q=span]
        [%cube p=* q=span]
    ==

This mold is not the entire definition of a Hoon span, just the cases we've seen so far. In English, a valid span is either:

- a cell with head `%atom`, and tail some symbol.

- a cell with head `%cell`, and tail some pair of spans.

- a cell with head `%cube`, and tail a noun-span pair.

I do think this is clear enough. I'm not sure it's intuitive enough. The following chapters should help with that; sorry.

But is this a representation of a mold or span? It seems to describe a set rather than define a function, so the latter seems more likely, but the section header implies otherwise.

That's pretty much what a mold is. The syntax is written to look as much as possible like the description of a set, but the semantics are that you're actually defining a function.

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