Thanks from a python programmer

[Preetpal]

On Friday, 5 March 2021 at 06:11:33 UTC, mipri wrote:
> On Friday, 5 March 2021 at 02:13:44 UTC, James Lu wrote:
>> On Thursday, 4 March 2021 at 20:14:32 UTC, Curious George 
>> wrote:
>>>
>>> I know this might be construed as rude to say this on D's NG, 
>>> but have you checked out nim? It has python-like syntax, can 
>>> compile directly to C, and thus utilize such libraries and 
>>> code easier than D can. It's also a growing language.
>>
>> Python has first-class C integration, and D has first-class 
>> extern(C) support.
>
> Here's a complete Nim program that prints the max amount of
> file descriptors that a select() set can have:
>
>   let FD_SETSIZE {.importc: "FD_SETSIZE", header: 
> "<sys/select.h>".}: cint
>   echo FD_SETSIZE
>
> Here's a version that exports a function that returns that
> value:
>
>   let FD_SETSIZE* {.importc: "FD_SETSIZE", header: 
> "<sys/select.h>".}: cint
>   proc setsize*: cint {.exportc.} = FD_SETSIZE
>
> With this assembly:
>
>   setsize:
>    endbr64
>    mov    eax,0x400
>    ret
>    nop    WORD PTR [rax+rax*1+0x0]
>
> So FD_SETSIZE is 1024 on godbolt.org. There's no 1024 in the
> Nim, and Nim is not parsing the C headers or anything; it's
> just deferring the value to C.
>
> This is a nice thing you get from a transpiler... which nobody
> using Nim really exploits, preferring to put a bunch of Nim
> const definitions with what the C headers have. Anyway, here's
> some more very uncommon Nim:
>
>   proc `*`(a: float, b: int64): float =
>      {.emit: [result, " = ", a, " * ", b, ";"].}
>
> That's a * operator that literally just returns whatever a*b
> does with those types in C.

I am very skeptical about the usefulness of transpiling in the 
manner you describe. Can you show some examples of this being 
used? How is it any different than just generating a header file 
in an intermediate build step (like how CMAKE can configure a 
version header file or you could even write a C program to write 
a header definition file)? Isn't this worse than just re-defining 
the constant, since now you lose compile time knowledge of what 
the constant should be?