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The other day, I saw a video on Brian Will’s YouTube channel — I’m afraid I don’t remember which video it was — in which he mentioned how cool it would be if a programming language offered a special kind of function that can only access the parameters it was given and nothing else. The function would then be referentially transparent and, therefore, truly modular: you could cut it out of any arbitrary code and paste it into any arbitrary code and it would still work.
I think Brian’s idea could be implemented relatively easily. The transpiler could check the generated abstract syntax tree for any symbols within the body of a function that are not part of that function’s argument list.
Having such functions seems to me a great step toward guaranteeing referential transparency. Alas, it’s not itself a complete guarantee yet, because you could still pass an impure function g that then gets invoked by your newly declared function f. If f‘s return value changes depending on g’s return value, f is not referentially transparent.
To solve this problem, you may choose to have the transpiler throw an exception whenever a function you declare to be referentially transparent takes referentially intransparent functions as arguments.
EDIT: This feature is now implemented as part of the Berlin programming language.
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