VIP: allow variable ranges with max bounds

[charles-cooper]

Simple Summary

Add variable ranges with max bounds, e.g. for i in range(<var>, bound=100)

Motivation

Right now vyper requires for-loop ranges to be bounded at compile time. However, an extremely common use case is to iterate over some variable counter at runtime. This leads to the common idiom,

for i in range(SOME_BOUND):
    if i >= self.count:
        break
    <loop body>

This proposal allows a runtime variable counter but also requires a max bound on the range.

Specification

Add a bound keyword arg to the range builtin, and relax the restrictions on the first argument to allow it to be a non-literal variable. For instance, the above example would become

for i in range(self.count, bound=SOME_BOUND):
    <loop body>

Backwards Compatibility

None known

Dependencies

References

Copyright

Copyright and related rights waived via CC0

[charles-cooper]

This probably becomes more relevant with dynamic arrays #1440

[fubuloubu]

Might be more interesting to do it like a slice of the dynamic variable itself:

for item in slice(self.dynamic_array, self.start, LENGTH):
    <loop body>

self.start += LENGTH  # So next time we do it, we start ahead of last time

[charles-cooper]

Yeah often we want access to the i value tho, so maybe we need enumerate(slice(...))

[fubuloubu]

Yeah often we want access to the i value tho, so maybe we need enumerate(slice(...))

I like enumerate

[fubuloubu]

Meeting notes: consider iteration over slice, and how it fits in with custom storage types #1954

[charles-cooper]

and also how it fits in with dynamic arrays #1440

[charles-cooper]

Implemented in #3537