Something about the way module names are specified in a file seems really strange to me. When I first started learning Haskell (I had used OCaml previously), I tried things like module Main where import A import B main = A.f >> B.f module A where f = ... module B where f = ... in a single file. This example is straight from chapter 5 of the Report, and no mention is made (that I could find) about modules needing to be in separate files. But this won't load in ghci! (Even if ... is changed to putStr "hi"). Eventually I figured out that it works fine if it's split over three separate files. So here's what I'm trying to figure out: If every file corresponds to exactly one module (is that true?), then why must the module name be given again in the text of the file? When I'm using ghci, I have lots of modules that I sometimes want to load "as Main", and sometimes I only want them loaded as a dependency from another module. Currently, I have to go into each file to change the "module Foo where" line to do this. Chad Scherrer
Scherrer, Chad wrote:
module Main where import A import B main = A.f >> B.f
module A where f = ...
module B where f = ...
in a single file. This example is straight from chapter 5 of the Report, and no mention is made (that I could find) about modules needing to be in separate files. But this won't load in ghci! (Even if ... is changed to putStr "hi"). Eventually I figured out that it works fine if it's split over three separate files.
The report says that they make up a single program, so that does not imply that they are in a single file. But, you are right: it's a good idea to be explicit about this.
So here's what I'm trying to figure out: If every file corresponds to exactly one module (is that true?), then why must the module name be given again in the text of the file? When I'm using ghci, I have lots of modules that I sometimes want to load "as Main", and sometimes I only want them loaded as a dependency from another module. Currently, I have to go into each file to change the "module Foo where" line to do this.
Section 9.5 of the report seems to show that <module> stands for the compilation unit and it defines a single module. I suppose the standard allows you to name your module freely no matter your filesystem allows, but ghc requires (afaik) that module M exist in M.hs or M.lhs. Why not do this: name none of those modules Main.hs, and have an empty module Main.hs with only "import MainDeJour" and "main = MainDeJour.main" so you can just edit just that file. Cheers, Koray
-----Original Message----- From: S Koray Can [mailto:skoraycan@aim.com] Why not do this: name none of those modules Main.hs, and have an empty module Main.hs with only "import MainDeJour" and "main = MainDeJour.main" so you can just edit just that file. Cheers, Koray ---------------------------------------------------------------------- Yeah, I like that approach. That saves me from having to remember which file I most recent used as main. Seems easy enough to even set it up so that load MainDuJour writes the file Main.hs with import MainDuJour main = MainDuJour.main and then and then calls ghc --make Main.hs -o mainDuJour This will do for now, but still feels really kludgy, especially for Haskell. -Chad
On Fri, Dec 16, 2005 at 07:55:50AM -0800, Scherrer, Chad wrote:
From: S Koray Can [mailto:skoraycan@aim.com] Why not do this: name none of those modules Main.hs, and have an empty module Main.hs with only "import MainDeJour" and "main = MainDeJour.main" so you can just edit just that file.
Cheers, Koray
---------------------------------------------------------------------- Yeah, I like that approach. That saves me from having to remember which file I most recent used as main. Seems easy enough to even set it up so that load MainDuJour writes the file Main.hs with
import MainDuJour main = MainDuJour.main
A rather late reply I realize, but this slightly less verbose version also works:
module Main where
import MainDuJour
Remi
Yes, good point. I suppose there's really no need to re-declare main once it's been imported. Thanks! -----Original Message----- From: Remi Turk [mailto:rturk@science.uva.nl] Sent: Thu 12/29/2005 4:34 PM To: Scherrer, Chad Cc: S Koray Can; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] RE: module names On Fri, Dec 16, 2005 at 07:55:50AM -0800, Scherrer, Chad wrote:
From: S Koray Can [mailto:skoraycan@aim.com] Why not do this: name none of those modules Main.hs, and have an empty module Main.hs with only "import MainDeJour" and "main = MainDeJour.main" so you can just edit just that file.
Cheers, Koray
---------------------------------------------------------------------- Yeah, I like that approach. That saves me from having to remember which file I most recent used as main. Seems easy enough to even set it up so that load MainDuJour writes the file Main.hs with
import MainDuJour main = MainDuJour.main
A rather late reply I realize, but this slightly less verbose version also works:
module Main where
import MainDuJour
Remi
Hello S, Friday, December 16, 2005, 6:32:47 AM, you wrote: SKC> Why not do this: name none of those modules Main.hs, and have an empty SKC> module Main.hs with only "import MainDeJour" and "main = SKC> MainDeJour.main" so you can just edit just that file. ghc has option "-main-is" which is just what needed in this case -- Best regards, Bulat mailto:bulatz@HotPOP.com
"Scherrer, Chad"
module Main where module A where module B where
in a single file. This example is straight from chapter 5 of the Report, and no mention is made (that I could find) about modules needing to be in separate files. But this won't load in ghci!
IIRC, the standard deliberately doesn't specify any requirements as to the file organization of code, and leaves it up to implementations. I think the reason file names and module names must coincide (with the exception of module Main, which may be in an arbitrarily named file), is simply that it makes it easier for the compiler (and the programmer as well) to locate them. Perhaps now that we have hierarchical namespaces we could allow a "module directory" (i.e. any prefix up to a dot) to be represented either as a directory (like it is now), or as a multi-module file? -k -- If I haven't seen further, it is by standing in the footprints of giants
Scherrer, Chad writes:
When I'm using ghci, I have lots of modules that I sometimes want to load "as Main", and sometimes I only want them loaded as a dependency from another module. Currently, I have to go into each file to change the "module Foo where" line to do this.
Maybe the "-main-is" option can help to make your live easier? You'll find more information here: http://haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html#id31... Peter
participants (6)
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Bulat Ziganshin -
Ketil Malde -
Peter Simons -
Remi Turk -
S Koray Can -
Scherrer, Chad