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Fix Issue 21731 - SumType should provide convenient access to the typ… #7886

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dlang-bot merged 2 commits into from
Mar 20, 2021
Merged

Fix Issue 21731 - SumType should provide convenient access to the typ… #7886

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Fix Issue 21731 - SumType should provide convenient access to the typ…
pbackus Mar 19, 2021
e1baa9b
Reword documentation for SumType.typeIndex
pbackus Mar 20, 2021
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73 changes: 73 additions & 0 deletions std/sumtype.d
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Original file line number Diff line number Diff line change
Expand Up @@ -790,6 +790,18 @@ public:
return this.match!hashOf;
}
}

/**
* Returns the index of the type of the `SumType`'s current value in the
* `SumType`'s `Types`.
*
* If the `SumType` is qualified, returns the index of the type in `Types`
* whose qualified version matches the `SumType`'s current value.
*/
size_t typeIndex() const
{
return tag;
}
}

// Construction
Expand Down Expand Up @@ -1466,6 +1478,67 @@ version (D_BetterC) {} else
MySum y; y = b;
}

// Type index
@safe unittest
{
alias MySum = SumType!(int, float);

static bool isIndexOf(Target, Types...)(size_t i)
{
switch (i)
{
static foreach (tid, T; Types)
case tid: return is(T == Target);
default: return false;
}
}

assert(isIndexOf!(int, MySum.Types)(MySum(42).typeIndex));
assert(isIndexOf!(float, MySum.Types)(MySum(3.14).typeIndex));
}

// Type index for qualified SumTypes
// Disabled in BetterC due to use of dynamic arrays
version (D_BetterC) {} else
@safe unittest
{
alias MySum = SumType!(const(int[]), int[]);

static bool isIndexOf(Target, Types...)(size_t i)
{
switch (i)
{
static foreach (tid, T; Types)
case tid: return is(T == Target);
default: return false;
}
}

int[] ma = [1, 2, 3];
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@Geod24 Geod24 Mar 19, 2021

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Suggested change
int[] ma = [1, 2, 3];
int[3] ma = [1, 2, 3];

Wouldn't that be enough to make it work in betterC ?

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@pbackus pbackus Mar 19, 2021

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In theory, yes. In practice, if I use a stack-allocated static array and construct the SumType with a slice (i.e., MySum(ma[])), the tests will fail even with -preview=dip1000, because the workaround for issue 21229 used in SumType's constructors does not correctly preserve lifetime information about the constructor's argument.

// Construct as mutable and convert to const to get mismatched type + tag
auto x = MySum(ma);
const y = MySum(ma);
auto z = const(MySum)(ma);

assert(isIndexOf!(int[], MySum.Types)(x.typeIndex));
assert(isIndexOf!(const(int[]), Map!(ConstOf, MySum.Types))(y.typeIndex));
assert(isIndexOf!(const(int[]), Map!(ConstOf, MySum.Types))(z.typeIndex));
}

// Type index for differently-qualified versions of the same SumType
// Disabled in BetterC due to use of dynamic arrays
version (D_BetterC) {} else
@safe unittest
{
alias MySum = SumType!(const(int[]), int[]);

int[] ma = [1, 2, 3];
auto x = MySum(ma);
const y = x;

assert(x.typeIndex == y.typeIndex);
}

/// True if `T` is an instance of the `SumType` template, otherwise false.
private enum bool isSumTypeInstance(T) = is(T == SumType!Args, Args...);

Expand Down