spec: use (*[4]int)(x) to convert slice x into array pointer

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spec: use (*[4]int)(x) to convert slice x into array pointer #395

Closed rogpeppe closed 3 years ago

rogpeppe commented 14 years ago

Currently, it's possible to convert from an array or array pointer to a slice, but
there's no way of reversing 
this.

A possible syntax could be similar to the current notation for type assertions:

ArrayAssertion  = "." "[" Expression ":" Expression
"]" .

where the operands either side of the ":" are constant expressions.

One motivation for doing this is that using an array pointer allows the compiler to
range check constant 
indices at compile time.

a function like this:

func foo(a []int) int
{
    return a[0] + a[1] + a[2] + a[3];
}

could be turned into:

func foo(a []int) int
{
    b := a.[0:4];
    return b[0] + b[1] + b[2] + b[3];
}

allowing the compiler to check all the bounds once only and give compile-time errors
about out of range 
indices.

bcmills commented 3 years ago

Personally I'd prefer that we use the length of the slice. If a[i] would panic for the original slice, then I think (*A)(a)[i] should either panic or be statically disallowed.

josharian commented 3 years ago

the length of the slice must equal the length of the pointed-to array

Why == instead of >=? The conversation above has pretty consistently discussed >=.

mdempsky commented 3 years ago

To simplify measuring opinions here, I recommend:

On Bryan's last comment (2 above this one), vote:
- :+1: if the conversion should be allowed based on the slice's length
- :-1: if the conversion should be allowed based on the slice's capacity
On Josh's last comment (1 above this one), vote:
- :+1: if the conversion test should be compared using >=
- :-1: if the conversion test should be compared using ==

josharian commented 3 years ago

Seems like there are some clear trends. :) I'll update the CLs.

nightlyone commented 3 years ago

@josharian I didn't see any release notes related to this. Could you make sure any of the changes get a RELNOTES=yes Tag so it won't be forgotten?

josharian commented 3 years ago

@nightlyone done. (None of them have been submitted yet, waiting on reviews.)

betawaffle commented 3 years ago

What about allowing conversion from *T to *[1]T as well? I would find that quite useful when trying to call a function that accepts a slice, but I have a single item already allocated from elsewhere.

bcmills commented 3 years ago

@betawaffle, that's an interesting idea, but this proposal was already approved as-is — want to file a separate proposal for converting between *T and *[1]T?

betawaffle commented 3 years ago

@bcmills: Thanks for the suggestion. Opened #45545.