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The Grug Brained Developer (2022)

(grugbrain.dev)
1087 points smartmic | 1 comments | 17 Jun 25 20:24 UTC | HN request time: 0.203s | source
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12_throw_away ◴[17 Jun 25 21:01 UTC] No.44303909[source]▶
This has by far the best discussion of the visitor pattern I've yet to come across.
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dgb23 ◴[17 Jun 25 21:47 UTC] No.44304309[source]▶
I don't work in typical OO codebases, so I wasn't aware of what the visitor pattern even is. But there's an _excellent_ book about building an interpreter (and vm) "crafting interpreters". It has a section where it uses the visitor pattern.

https://craftinginterpreters.com/representing-code.html#the-...

I remember reading through it and not understanding why it had to be this complicated and then just used a tagged union instead.

Maybe I'm too stupid for OO. But I think that's kind of the point of the grug article as well. Why burden ourselves with indirection and complexity when there's a more straight forward way?

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12_throw_away ◴[17 Jun 25 22:32 UTC] No.44304698[source]▶
As far as I understand it, the limited circumstances when you absolutely need the visitor pattern are when you have type erasure, i.e., can't use a tagged union or its equivalent? In that case visitors are AIUI a very clever trick to use vtables or whatever to get back to your concrete types! but ... clever tricks make grug angry.
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1. zem ◴[17 Jun 25 23:53 UTC] No.44305249[source]▶
even when you have tagged unions, visitors are a useful way to abstract a heterogenous tree traversal from code that processes specific nodes in the tree. e.g. if you have an ast with an `if` node and subnodes `condition`, `if_body`, and `else_body` you could either have the `if node == "if" then call f(subnode) for subnode in [node.condition, node.if_body, node.else_body]` and repeat that for every function `f` that walks the tree, or define a visitor that takes `f` as an argument and keep the knowledge of which subnodes every node has in a single place.