lazy import, an alternative to DIP 1005

Jonathan Marler via Digitalmars-d digitalmars-d at
Fri Sep 15 14:45:01 UTC 2017

DIP 1005 proposes a solution to prevent loading in modules that 
don't need to be loaded, thereby decreasing the overall compile 
time. Here's an example taken from the DIP:

with (import std.stdio) void process(File input) ;
with (import std.range) struct Buffered(Range) if 

I've come up with another way to solve this problem by using a 
modified version of import, call it a "lazy import".  A lazy 
import works like a "static import" in that all references to it 
must be fully qualified, but it also delays loading the module 
until absolutely necessary.  Since all the symbols will be fully 
qualified the semantic analyzer will know when it's time to load 
the module. The previous example could be done like this:

lazy import std.stdio;
lazy import std.range;

void process(std.stdio.File input) ;
struct Buffered(Range) if (std.range.isInputRange!Range)

Note that instead of introducing the "lazy" modifier here, we 
could just modify static imports to be lazy which would mean no 
new syntax and every benefits from the feature without changing 
their code, that is, if they are using static imports :)

aliasing can also be helpful in this instance

alias stdio = std.stdio;
alias range = std.range;

void process(stdio.File input) ;
struct Buffered(Range) if (range.isInputRange!Range)

Note that this usage is only meant for large library code like 
phobos, where this type of optimization can have a big 
performance impact when importing modules from the library.  You 
wouldn't use the verbose syntax from either of the examples in a 
normal application.

What do you think, better or worse than DIP 1005?  I think it's 
simpler, but not sure if it's better overall.

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