Document `%transform`

[andrevidela]

%transform has been a successful part of the compiler for some time now and I couldn't find any good reference for how it is supposed to work and what it is supposed to do. The only page I could find that mentions it is https://idris2.readthedocs.io/en/latest/reference/builtins.html?highlight=transform#builtin-integertonatural

Shouldn't there be a more descriptive tutorial about %transform?

[gallais]

Note that %transform is not mature enough to express e.g. fusion rules. So whatever doc we write now will have to be rewritten once it's fixed.

[andrevidela]

One could argue that the fusion rules should be replaced by linearity optimisations :)

[gallais]

I'm not sure I understand what you're saying. Fusion turns two (or more) traversals into a single one. It's not just about allocation.

[andrevidela]

I haven't looked into this recently but I was under the impression that Fusion could be replaced by deforestation allowing you to ensure the traversals are properly inlined and merged. The problem with fusion being that its heuristics don't always predictably trigger, but relying on linear use makes a non-optimised tree traversal a type error