On 28 November 2012 14:39, Marius Gundersen <[email protected]> wrote: > On Wed, Nov 28, 2012 at 1:20 PM, Andreas Rossberg <[email protected]> > wrote: >> First, your definition of "pure" is not quite correct. Any function >> that even _returns_ locally created state in some form (i.e., a new >> object), is impure. > > Fair enough. A better name would probably be side effect free functions.
Well, observable allocation of state, or assignment to parameters, _are_ side effects. On the other hand, accessing non-local bindings that are (deeply) immutable does not constitute an effect, yet you want to forbid it. Seriously, what is the use case for this rather strange definition? >> Last, short of a static type-and-effect system, I don't see how the >> necessary checks could be implemented without imposing significant >> overhead on almost every primitive operation -- because every >> function, and hence almost any piece of code, might potentially end up >> with a "pure" function in its call chain, and would need to check for >> that. > > I'm not an implementer of EcmaScript, so I don't have deep knowledge of how > this could be implemented. I would imagine that a subset of a true pure > function, where the only restriction would be that only variables passed as > arguments to the function exist in the scope, would be relatively easy to > implement. Wouldn't this be a faster implementation than todays functions, > which have to keep track of scope? These side-effect-free functions would > only need to contain the variables passed as parameters in their scope. > Accessing anything outside the scope would result in a reference error. Modern JS implementations don't do the kind of runtime bookkeeping of scopes that you seem to assume. Compiled code doesn't even know what a scope is, it just accesses hardcoded indices into the stack or into some heap arrays. /Andreas _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
