My question is, why throw is a function in Julia? Why is it not a keyword just like the case in many other languages?
The short answer it that keywords are only used when necessary.
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That’s the answer I expect (and agree with). Still, the question is now “to what extent can throw be regarded as a function”. For example, how can throw(which changes the control flow) be implemented using some other functions?
mythrow(x) = ccall(:jl_throw, Cvoid, (Any,), x)
Note that since it’s not a tfunc like throw, it doesn’t have the same type inference support, so in some cases, this might result in worse type inference than throw.
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To the extent that it obeys the semantics of a function, ie you can call it with arguments.
Not without reaching for internals, it is a built-in function.
That said, I am not really sure what the purpose of this discussion is though. Like all languages, Julia needs a primitive to throw exceptions, it has throw, and it works fine.
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One reason is that many static languages have no multiple dispatch, so they can’t have a polymorphic throw function, since there are many different types of errors. But Haskell indeed has a throw function, and its type signature is quite complicated.
For some dynamic languages like Python, using a keyword can prevent you redefining the function accidentally. So it’s illegal to write down raise=1 or throw=print…
Though strictly speaking Julia’s throw is a built-in function, not a generic function. But at least it acts like a generic function.
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That makes me think if could be a small macro instead of a keyword. for, too. That would make the language simpler and things like symbolic execution easier.
My 2c. I think it makes much more sense for throw to be a built-in function in Julia. There are also other built-in functions which are impossible to implement without accessing internal functions (e.g. getfield and setfield!). If you need a method, you can wrap these functions in some custom function (e.g. getproperty and setproperty!).
I understand why other languages implement throw as a keyword, but I don’t think it makes sense to do that in Julia. Take Scheme languages, for instance, there are a lot of them which implement Call/cc, which changes control flow in a much more complex way than Julia’s throw (or any language’s throw keyword) and they are still functions.
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