How can we make it easier to experiment with the compiler?

[Walter Bright]

On 5/23/2021 7:25 PM, Nicholas Wilson wrote:
> Directories.

The #1 problem isn't directories, it's "every module imports every other module" 
that leaves one with nowhere to start.

We currently have:

   dmd
   dmd/root
   dmd/backend

I regularly fend off attempts to have dmd/root import files from dmd, and 
dmd/backend import files from dmd. I recently had to talk someone out of having 
dmd/backend import files from dmd/root.

In other words, a failure of encapsulation.

Let's look at one example, picked more or less because I've looked at it 
recently, dmd/target.d. The reason for its existence is to abstract target 
information. It's imports are:

   import dmd.argtypes_x86;
   import dmd.argtypes_sysv_x64;
   import core.stdc.string : strlen;
   import dmd.cond;
   import dmd.cppmangle;
   import dmd.cppmanglewin;
   import dmd.dclass;
   import dmd.declaration;
   import dmd.dscope;
   import dmd.dstruct;
   import dmd.dsymbol;
   import dmd.expression;
   import dmd.func;
   import dmd.globals;
   import dmd.id;
   import dmd.identifier;
   import dmd.mtype;
   import dmd.statement;
   import dmd.typesem;
   import dmd.tokens : TOK;
   import dmd.root.ctfloat;
   import dmd.root.outbuffer;
   import dmd.root.string : toDString;

If I want to understand the code, I have to understand half of the rest of the 
compiler. On a more abstract level, why on earth would a target abstraction need 
to know about AST nodes? At least half of these imports shouldn't be here, and 
if they are, the code needs to be redesigned.

Recently I needed some target information in the ImportC lexer, and it would 
have been so easy to just import dmd.target. But then that drags along all the 
imports that I've really tried to avoid importing into the lexer.

Iain came up with a clever solution to use a template parameter.

Note that Phobos suffers terribly from this disease (everything ultimately 
imports everything else), which makes it very hard to understand and debug.

Fixing this is not easy, it requires a lot of hard thinking about what a module 
*really* needs to do. But each success at eliminating an import makes it more 
understandable.

Creating a false hierarchy (an implied relationship that is instantly defeated 
by the imports) of files won't fix it.

A good rule of thumb is:

     *** Never import a file from an uplevel directory ***

Import sideways and down, never up.