Can std.variant be used with std.container.rbtree? (page 2)

April 02, 2022

Re: Can std.variant be used with std.container.rbtree?

Posted by Paul Backus
in reply to Vijay Nayar

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Paul Backus

Posted in reply to Vijay Nayar

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On Saturday, 2 April 2022 at 14:49:15 UTC, Vijay Nayar wrote:

On Saturday, 2 April 2022 at 14:35:10 UTC, Vijay Nayar wrote:

The tryMatch method fails to compile, and instead I get the following error:

/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(2004): Error: static assert:  "`handlers[0]` of type `int function(ref ubyte[] _1, ref ubyte[] _2) pure nothrow @nogc @safe` never matches"

Through sheer trial and error, I discovered that changing the handler arguments from ref ubyte[] to const ref ubyte[] the error goes away. However, I did not find any information in the documentation or code that made this requirement clear. It was basically a guess based on the fact that string has no trouble but ubyte[] did, and string is basically just immutable(char[]).

The reason const is required is that the arguments to your function, v1 and v2, are const. In D, the const qualifier is transitive, so if you have a const VarType, whatever value it contains will also be qualified with const.

So far, I'm finding that learning to use SumType is significantly more cryptic than Variant, by a large margin. I'll still give it a shot of course, because I want to get past this problem.

It's possible to get more informative error messages using the -verrors=spec compiler flag, although it's a bit of a hassle. The basic process is

Recompile the module that the error occurs in with -verrors=spec and redirect output to a file.
```
dmd -verrors=spec -c source/mymodule.d >verrors.txt 2>&1
```
Use grep or your text editor's search function to find errors that occurred in that source file or in sumtype.d.
```
grep -e 'source/mymodule.d' -e 'std/sumtype.d' verrors.txt
```

Following these steps on your example code yields around 20 lines of output, which includes the "real" error message:

(spec:1) /usr/include/dmd/phobos/std/sumtype.d(1768): Error: function literal `__lambda3(ref ubyte[] _1, ref ubyte[] _2)` is not callable using argument types `(const(ubyte[]), const(ubyte[]))`
(spec:1) /usr/include/dmd/phobos/std/sumtype.d(1768):        cannot pass argument `_param_0` of type `const(ubyte[])` to parameter `ref ubyte[] _1`

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