How to assign and compare arrays to SumType?

Jun 11

confuzzled

Jun 11

confuzzled

Jun 11

Jun 11

Jun 11

Jun 11

import std.variant; import std.sumtype; import std.stdio; struct S { SumType!(double[]) data; // {1} } void main() { Variant v = [1.7, 2.7, 3.7, 4.7, 5.7]; assert(v == [1.7, 2.7, 3.7, 4.7, 5.7]); S s; s.data = [1.7, 2.7, 3.7, 4.7, 5.7]; // {2} assert(s.data == [1.7, 2.7, 3.7, 4.7, 5.7]); }

s.data = [SumType!double(1.7), SumType!double(2.7), SumType!double(3.7), SumType!double(4.7), SumType!double(5.7)]; assert(s.data == [SumType!double(1.7), SumType!double(2.7), SumType!double(3.7), SumType!double(4.7), SumType!double(5.7)]);

import std.variant; import std.sumtype; import std.traits: isArray; struct S { SumType!(double[]) data; bool opEquals(T)(auto ref const T s) const if (isArray!T) { return data == typeof(data)(s); } void opAssign(T)(T value) if (isArray!T) { data = typeof(data)(value); } this(T)(T value) if (isArray!T) { opAssign(value); } } void main() { Variant v = [1.7, 2.7, 3.7, 4.7, 5.7]; assert(v == [1.7, 2.7, 3.7, 4.7, 5.7]); S s = [1.7, 2.7, 3.7, 4.7, 5.7]; assert(s == [1.7, 2.7, 3.7, 4.7, 5.7]); }

void main() { Variant v = [1.7, 2.7, 3.7, 4.7, 5.7]; assert(v == [1.7, 2.7, 3.7, 4.7, 5.7]); S s; s.data = [1.7, 2.7, 3.7, 4.7, 5.7]; // {2} assert(s.data == SumType!(double[])([1.7, 2.7, 3.7, 4.7, 5.7])); }

June 11

Re: How to assign and compare arrays to SumType?

Posted by Steven Schveighoffer
in reply to confuzzled

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Steven Schveighoffer

Posted in reply to confuzzled

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On Tuesday, 11 June 2024 at 16:41:46 UTC, confuzzled wrote:

Comparison between a Variant and an array is straightforward. How does one accomplish the same between a SumType and an array?

import std.variant;
import std.sumtype;
import std.stdio;

struct S
{
    SumType!(double[]) data;  // {1}
}

void main()
{
    Variant v = [1.7, 2.7, 3.7, 4.7, 5.7];
    assert(v == [1.7, 2.7, 3.7, 4.7, 5.7]);

    S s;
    s.data = [1.7, 2.7, 3.7, 4.7, 5.7]; // {2}
    assert(s.data == [1.7, 2.7, 3.7, 4.7, 5.7]);
}

Resulting Error:

var.d(17): Error: template `opEquals` is not callable using argument types !()(double[])
/Users/anju/dlang/dmd-2.109.0-beta.1/osx/bin/../../src/phobos/std/sumtype.d(712): Candidate is: opEquals(this This, Rhs)(auto ref Rhs rhs)
with This = SumType!(double[]), Rhs = double[]
must satisfy the following constraint:
!is(CommonType!(This, Rhs) == void)

Also, assuming that {1} read "SumType!(double)[] data;", what would be the proper way to accomplish the assignment at {2} and the subsequent comparison.

SumType requires you unwrap to the correct type in order to use it.

The function that unwraps a SumType is called match. The match function accepts a sumtype and a list of processors. Each processor is tried in order, based on the actual stored type, and the first one that compiles will be used.

Your SumType has only one type, but just to illustrate how it would work:

assert(s.data.match!((double[] darr) => darr == [1.7, 2.7, 3.7, 4.7, 5.7]));

What this is doing is, if the actual type is a double[], then call the first function, and return it's value (a boolean).

Because there are no other types, this will work. If you have more than one type, your list of lambdas must be exhaustive. That is, at least one of the functions must match for each type. And also, functions that won't ever be used are rejected.

Let's see how it works if you want to both check that the value is a double[] and that it matches your value:

SumType!(double[], int) s = [1.7, 2.7, 3.7, 4.7, 5.7];
assert(s.data.match!(
        (double[] darr) => darr == [1.7, 2.7, 3.7, 4.7, 5.7]),
        _ => false // any other type
    )
);
s = 5; // switch to int
assert(!s.data.match!(
        (double[] darr) => darr == [1.7, 2.7, 3.7, 4.7, 5.7]),
        _ => false // any other type
    )
);

SumTypes take some getting used to. It can get pretty verbose to use match.

-Steve

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