proposal: spec: type parametrized array lengths

[soypat]

Proposal Details

Context

Today there is no way to allow parametrization of array lengths in types other than using constant declarations like so

const N = 100 // Parametrization defined at package level.

type RingBuffer struct {
   off, end int
   buf      [N]byte
}

Drawbacks

Poor parametrization for module users: Note the length of the array is defined at a package level and when the type is exported it cannot be changed by end user other than using build tags which must be defined beforehand by module provider.

Poor reusability: Even if parametrization is allowed it can only be specialized once per compile. Regardless of build tag customization you need to create a new type for each array length desired. You also need to rewrite each method from scratch.

Proposal

Add possibility of array length type parametrization.

Example (demonstrative purposes only):

func NewRingBuffer[N [N]byte]() RingBuffer[N] {
      return RingBuffer[N]{}
}

const buflen = 500
r1 := NewRingBuffer[500]()
r2 := NewRingBuffer[buflen]()

func getArrayPtr[N [N]byte](buf []byte, off int) *[N]byte {
    return (*[N]byte)(frm.buf[off:N+off])
}

dataptr := getArrayPtr[20](r1.buf, 10) 

Author's note (more context)

I use Go in embedded systems with TinyGo. My concern is memory usage regarding constraint on size and GC overhead and memory fragmentation. I'm currently writing a networking stack which can run on 100kB of RAM. I regularly make use of arrays to avoid heap allocations which are costly and they also fragment memory on these small devices. This feature would help the embedded scene a bunch for writing low level drivers :)

Below are links to OSI layers I have implemented in TinyGo.

• Layer 1: 3 wire SPI interface on Raspberry Pi Pico
• Layer 2: interfacing with CYW43439 wifi driver over 3 wire SPI
• Layer 3: Networking stack written in pure Go implementing Ethernet, IPv4, DHCP, DNS, NTP framing, handling and routing
• Layer 4, 5: TCP implementation in Pure Go (also part of seqs)
• Layer 6: Lightweight HTTP implementation .

[gabyhelp]

Related Issues

• proposal: spec: generic parameterization of array sizes #44253
• proposal: Go 2: Const generics #65555 (closed)

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[ianlancetaylor]

Thanks. Closing as dup of #44253.

[soypat]

@ianlancetaylor Whoops- should I add a comment with my proposal to 44253?

Edit: done!