Template Expression Spec Clarification

[Jes-s-L]

Hi, I have a few questions to clarify how multiple expressions in Template Expression Spec are handled.

• Are expression trees for each equation maintained independently?
• If so, are mutations/crossovers only performed on a single expression tree? i.e., do mutations/crossovers ever occur between separate expressions (both during each cycle and during migration)?
• In general, are there any other differences between how multiple expressions are handled vs. a single expression tree?

Thank you!

[MilesCranmer]

Hi @Jes-s-L,

Sorry, I completely missed this! Answers below:

• Are expression trees for each equation maintained independently?

A "template expression" is stored in memory as a list of expression trees, basically. So I guess the answer is no, they are actually maintained as a single object and carried around as one group of trees.

• If so, are mutations/crossovers only performed on a single expression tree? i.e., do mutations/crossovers ever occur between separate expressions (both during each cycle and during migration)?

The way it works is that a mutation or crossover will randomly pick one of the subtrees, and then dispatch to the normal mutation/crossover operation. So this means that at the moment, yes, you can definitely have crossovers between an expression for f(#1) and expression for g(#1). There aren't any types of mutations that are actually "aware" of the fact that it is a template expression; they simply pick a subtree and go from there.

• In general, are there any other differences between how multiple expressions are handled vs. a single expression tree?

Not really! It gets passed to all the same functions as a normal single expression tree. The AbstractExpression type in DynamicExpressions.jl is a very modular design which sets up a lot of this to "just work" automatically.

[Jes-s-L]

That makes sense, thank you for the detailed answers!