Behavior and non-behavior

[cchannon]

I keep getting stymied by these "Behavior function in a non-behavior property" errors as I am trying to build my own PFX implementation. In general, I get the idea: there are properties that expect an output and not an operation. and there are operations that expect actions (or behavior) and do not expect (or, at least, do not require) an output.

But I am having a hard time wrapping my head around how the Eval method is determining which expressions are and are not non-behavior properties. Sometimes I get this error on statements as simple as "Set(avariable, 1)" and I find it hard to imagine how that could be seen as a "non-behavior property". Is there a reference page somewhere that can help me understand this? The core documentation wiki reads kinda like a CS textbook (no offense!) so it is hard to figure out what I am looking for there.

[anderson-joyle]

A behavior function is a function that changes the value of a variable, or database table, or any other value that is somewhere previously stored. Most of Power Fx formulas (non-behavior) receive 0..N arguments, return a calculated value, and that's the end of it. The behavior function result, in the other hand, persists.
Currently there are mainly 4 behavior functions: Set, Collect, Patch, and Remove.
To enable the behavior functions, you got to explicitly "tell" the engine to. Please check the following test cases:
https://github.com/microsoft/Power-Fx/blob/main/src/tests/Microsoft.PowerFx.Interpreter.Tests/MutationFunctionsTests.cs

Take a closer look at these two lines:

line 29
private readonly ParserOptions _opts = new ParserOptions { AllowsSideEffects = true };

line 37
symbol.EnableMutationFunctions();

[cchannon]

OK, I am tracking this, and yes, these changes definitely smooth out the wrinkles in my tests, but I don't actually understand these changes yet. What does it actually mean to "Allow Side Effects"? And are "mutation functions" synonymous with Behavior functions, or is there a difference there?

[anderson-joyle]

Mutation is synonymous with Behavior.
"Allow Side Effects" means 2 things:

• Functions are allowed to change value from a source.
• Allows formula chaining (semicolon operator) e.g. "Set(a,10);a*2"