`pure`/`impure` modifiers for functions

[lars-reimann]

Is your feature request related to a problem?

Currently, we use annotations to denote the purity of functions. Since the information is vital for our execution concept, the use of modifiers for this purpose makes sense.

Desired solution

pure fun f() to denote a pure function.
impure fun g() to denote an impure function.

Remaining question: How do we denote the reasons for impurity? For a start, it suffices to denote

• file reads with path given by constant
• file reads with path given by parameter
• file writes with path given by constant
• file writes with path given by parameter.

Possible alternatives (optional)

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Additional Context (optional)

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[lars-reimann]

I'd probably stick to annotations, since I can't come up with nicer special syntax for the impure case. With annotations, it could look like this:

@Pure

@Impure(allReasons=[
    ImpurityReason.FileReadFromConstant("some/path/to/some/file.csv"),
    ImpurityReason.FileReadFromParameter("path"),
    ImpurityReason.Other
])

We would then remove the annotation @NoSideEffects and instead look at the @Pure/@Impure annotations to figure out whether a function has side effects or not.