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This repository was archived by the owner on Aug 17, 2022. It is now read-only.
Fixed-length lists #146
Description
This is definately in the Could-Have category.
I have the following type defined in Rust:
type Ipv6Address = [u8; 16];As this proposal currently stands, I'll have to resort to either a variable length list,
(list u8)
or (and hold on to your hat)
(record
(byte_0 u8)
(byte_1 u8)
(byte_2 u8)
(byte_3 u8)
(byte_4 u8)
(byte_5 u8)
(byte_6 u8)
(byte_7 u8)
(byte_8 u8)
(byte_9 u8)
(byte_10 u8)
(byte_11 u8)
(byte_12 u8)
(byte_13 u8)
(byte_14 u8)
(byte_15 u8)
)
Either one will work, but neither one pleasantly captures the intent.
Suggestion: add an optional length annotation to lists:
(list <intertype> (length <size>)?)
where <size> is an exact size. The list can never be shorter or longer. This might enable additional optimizations in the glue code.
I'm aware my specific IPv6 example could also be described using a pair of u64s, but that isn't a generic solution.
Activity
lukewagner commentedon Jan 26, 2022
Hm, yeah, I see your point. I suppose we could think of fixed-length lists as another specialization (e.g., as
optionalis a specialization ofvariant).rossberg commentedon Jan 26, 2022
I'd say this is in the same category as not having e.g. integer subrange types. If you know your value range of a parameter is 0..9999, you still have to use u16 or something (or a quadruple of decimal digits :) ). You can't express intent more specifically.
The reason low-level languages like C or Rust include the array size in their types is mostly because their representations depend on it. In higher-level languages that is not typically the case and the size it is not normally included. And I would count interface types among those higher-level type systems.
Attempts to allow capturing domain-specific semantics beyond mere representation are a huge can of worms for a type system, because there will always be the next case that you cannot express – e.g., what if the length of the list is given by another parameter? There are solutions to this, but none that you'd want to put into a cross-language IDL.