Re: [Haskell-cafe] Cyclic Inclusions

On 13 Aug 2008, at 13:18, ajb@spamcop.net wrote:

G'day all.

Quoting Thomas Davie :

...

To be honest, ghc compiles things so fast (at least on any of my systems) that I couldn't care less if it took 10 times as long (I would however like some added convenience for that time spent)

Have you ever compiled GHC itself? Just curious what you'd think about a 10x speed hit there. On the machine I'm sat at now, it takes 20 minutes. Secondly, you would *only* pay the speed penalty when you had cyclic includes, and you didn't use the flag to use an hi-boot file, so it would continue to take 20 minutes.

If it helps, think about the lifetime of a program. If you assume that a program grows linearly over time, and that recompilations occur at a roughly constant rate, it follows that the time spent recompiling is O(n^2). Constant factors matter. But the penalty would be no where near that big -- unless you happened to have *all* your modules depend on *all* other modules.

...

If I compile a module on which lots of other modules depend, I have to do lots of recompilation... If I compile a module which is in a cyclic dependancy group, I have to do lots of recompilation, I don't see that there's a difference here.

If you only change the implementation of a module, not its interface, you don't need to recompile anything that imports it. (At least, this is true at -O0, which if you care about fast recompilation because you're deep in development, you're probably doing.)

Well, if this turned out to take a long time, then I'd probably switch to writing an hi-boot file... Something that I wouldn't have to waste time doing at all if it happened that ghc was still "fast enough" without providing one. Bob