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digitalmars.D.learn - class grammar

reply Nick <my_dlang bestmail.us> writes:
The class grammar, as defined in the D language specification 
([Classes](https://dlang.org/spec/grammar.html#classes)), seems 
to imply that a class can inherit from a fundamental type. 
Explicitly, the specification says that a 'SuperClass' is a 
'BasicType'. And a 'FundamentalType' is a 'BasicType'.

However, when I attempt to compile the following declaration, I 
get an error.

```d
class A : int {}
```

```d
Error: class `test.A` base type must be `class` or `interface`, 
not `int`
```

After reading the grammar for a 'BasicType' 
([BasicType](https://dlang.org/spec/type.html#BasicType)), it is 
not clear to me how a user-defined type (such as a class or 
interface) is also a 'BasicType' (as defined by the grammar). 
However, the compiler only accepts classes or interfaces as a 
base type.

How am I misunderstanding the specification?
May 07 2021
next sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 7 May 2021 at 18:07:45 UTC, Nick wrote:

 The class grammar, as defined in the D language specification 
 ([Classes](https://dlang.org/spec/grammar.html#classes)), seems 
 to imply that a class can inherit from a fundamental type. 
 Explicitly, the specification says that a 'SuperClass' is a 
 'BasicType'. And a 'FundamentalType' is a 'BasicType'.


I think the grammar allows it to parse, but then defers the error 
till later.

There's a lot of things that are too generic for the parser to 
reject, but don't match the semantics later.

Like it the grammar only allowed a ClassOrInterface... it'd have 
to know what `X` is before it is parsed which is problematic.


 Error: class `test.A` base type must be `class` or `interface`, 
 not `int`


Notice that this is not a parse error, but rather a semantic 
check later on, which is consistent with that grammar.

The parser accepts it, then later on, a future pass of the 
compiler resolves what the word actually means and then says 
"wait a bit that is a sentence sure, but it doesn't actually make 
sense".
May 07 2021
prev sibling parent reply Basile B. <b2.temp gmx.com> writes:
On Friday, 7 May 2021 at 18:07:45 UTC, Nick wrote:

 The class grammar, as defined in the D language specification 
 ...
 is not clear to me how a user-defined type (such as a class or 
 interface) is also a 'BasicType' (as defined by the grammar). 
 However, the compiler only accepts classes or interfaces as a 
 base type.


Just like a natural language what is grammatically correct is not 
necessarily semantically correct.

In the case of the inherithence list it's easier to choose 
'BasicType' because 'TypeOf' and 'QualifiedIdentifier' can both 
be solved to a class or an interface.

With this example:

```
alias B = int;
class A : B {}
```

you can see that the semantic check to reject `int` is necessary 
anyway.
May 07 2021
parent Nick <my_dlang bestmail.us> writes:
On Friday, 7 May 2021 at 19:33:30 UTC, Basile B. wrote:

 On Friday, 7 May 2021 at 18:07:45 UTC, Nick wrote:

 The class grammar, as defined in the D language specification 
 ...
 is not clear to me how a user-defined type (such as a class or 
 interface) is also a 'BasicType' (as defined by the grammar). 
 However, the compiler only accepts classes or interfaces as a 
 base type.


 Just like a natural language what is grammatically correct is 
 not necessarily semantically correct.

 In the case of the inherithence list it's easier to choose 
 'BasicType' because 'TypeOf' and 'QualifiedIdentifier' can both 
 be solved to a class or an interface.

 With this example:

 ```
 alias B = int;
 class A : B {}
 ```

 you can see that the semantic check to reject `int` is 
 necessary anyway.


Okay. So, although the base class list grammar is permissive, the 
restricted semantics of this list is indicated by its optional 
elements being explicitly named 'SuperClass' (class) and 
'Interface' (interface). I now see how the 'QualifiedIdentifier' 
and 'Typeof' grammar could apply if these semantics are taken 
into account.
May 07 2021