mixin issue

[Paul Backus]

On Wednesday, 29 November 2023 at 13:31:14 UTC, DLearner wrote:
> ```
> Error: found `End of File` when expecting `;` following 
> statement
> ```
>
> If an extra ; is added:
> ```
>    }(` ~ strStartPtr ~ `,` ~ strPLPtr ~ `);`;
> ```
>
> it works but doesn't seem correct.

This is an annoying limitation of the D compiler.

The root of the problem is that there is an ambiguity in D's 
grammar. When the compiler sees

     mixin(whatever);

...it cannot tell whether it's supposed to be a [Mixin 
Statement][1], or an [Expression Statement][2] that contains a 
[Mixin Expression][3].

If it's a Mixin Statement, then there should be a semicolon 
inside the mixin. If it's an Expression Statement with a Mixin 
Expression, then there shouldn't be a semicolon inside the mixin.

To resolve this ambiguity, the compiler (currently) *assumes* 
that it's always a Mixin Statement, which means that it will 
always require a semicolon inside the mixin.

As a result, it is impossible to write a string mixin that can be 
used as both a statement and an expression.

If you have an expression mixin and you would like to use it as a 
statement, you can work around this limitation by adding 
`cast(void)` in front of the mixin:

     cast(void) mixin(whatever);

This forces the compiler to parse the line as an Expression 
Statement containing a [Cast Expression][4], but does not 
otherwise change the meaning of the code.

[1]: https://dlang.org/spec/statement.html#mixin-statement
[2]: https://dlang.org/spec/statement.html#expression-statement
[3]: https://dlang.org/spec/expression.html#mixin_expressions
[4]: https://dlang.org/spec/expression.html#cast_expressions