LET allows you to introduce local bindings and use them to name intermediate results.
I’m trying out a new format with this post. I introduce the feature, explain its intended semantics, and give you examples to test your understanding before walking through how I implemented it. There’s also a place to pause and try implementing it yourself.
Hit after hit after hit with these posts
@john-kelly Happy to hear you’ve been enjoying the posts.
I ended up exploring and writing about three different variations of multiple-binding let:
let Expressions — all of the initializers are evaluated in the incoming environment, so sibling bindings cannot see one another.let Expressions — bindings are evaluated in source order, with each successful binding becoming available to the ones that follow it.let Expressions — source order no longer determines which bindings are available. Instead, the dependencies between bindings determine a valid evaluation order.LETPAR and LETSEQ were the two variations I originally planned to explore. LETDEP came out of asking what would happen if forward references were allowed, which led to dependency graphs, topological sorting, free-variable analysis, and some interesting lexical-scope edge cases.
LETDEP also ends up fairly close to Elm’s let semantics, since definitions are not constrained by source order.
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