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:
```d
/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
1. 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
```
2. 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`
```
