Im writing app in haskell and I would like to export some functions and datatypes to other files and then be able to use them in my main file.
How to do this ?
thanks for help
You could lay out your source code like so:
Main.hs
A/Module.hs
You need to specify in A/Module.hs which module it actually is; it has to be:
module A.Module where
...
In Main.hs, you import A.Module; all names are exported by default.
The Wikibooks page on Haskell modules would be a good starting point, or the relevant section of Learn You a Haskell (especially the "Making our own modules" part).
Related
I need to find out why some module gets included into compilation.
There is some class that should not be included and I think there are some unused imports or bad architecture that requires unnecessary imports. Is there a way to find which modules import some module, which modules import these modules that include this module, and so on, tracking that down to main class of application?
You could use -D dump-dependencies for this, in which case the compiler will generate two files that can be used to follow the dependency graph in both directions:
dump/<target>/.dependants.dump
dump/<target>/.dependencies.dump
There is also a handy online tool created by Mark Knol that helps a lot with analyzing these files. To answer the question "what does Array depend on?", you can just upload the two files and enter "array" into the search field:
Conveniently, the results are also clickable.
I've just came up with very simple idea: just delete this file and there will be compilation errors in places where this module is imported.
I am trying to create a Python package. You can have a look at my awful attempt here.
I have a module called imguralbum.py. It lives in a directory called ImgurAlbumDownloader which I understand is the name of the package -- in terms of what you type in an import statement, e.g.
import ImgurAlbumDownloader
My module contains two classes ImgurAlbumDownloader and ImgurAlbumException. I need to be able to use both of these classes in another module (script). However, I cannot for the life of me work out what I am supposed to put in my __init__.py file to make this so. I realize that this duplicates a lot of previously answered questions, but the advice seems very conflicting.
I still have to figure out why (I have some ideas), but this is now working:
from ImgurAlbumDownloader.imguralbum import ImgurAlbumDownloader, ImgurAlbumException
The trick was adding the package name to the module name.
It sure sounds to me like you don't actually want a package. It's OK to just use a single module, if your code just does one main thing and all its parts are closely related. Packages are useful when you have distinct parts of your code that might not all be needed at the same time, or when you have so much code that a single module would be very large and hard to find things in.
Suppose I have a Haskell module named MyModule that imports an external module like this:
import ModuleA hiding (a, b, c)
And I cannot modify this import statement, because the program is not exactly mine.
I wish to link to ModuleA.external_function in the documentation for ModuleA, in the comments above a function called my_function. So the code looks something like this:
-- | my_function makes use of 'ModuleA.external_function'
my_function :: Int -> Int
Using haddock 2.10.0, and running cabal haddock, the link to ModuleA.external_function is generated as dist/doc/html/MyModule/ModuleA.html#v:external_function . However, the problem is that the dist/doc/html/MyModule/ModuleA.html file does not exist.
How can I generate a link to the docs for ModuleA instead, like module-A-package/docs/ModuleA.html#v:external_function. In other words, something similar to what http://hackage.haskell.org/package/text-0.11.2.0/docs/Data-Text.html has for its links to the String type (they link to http://hackage.haskell.org/package/base-4.5.0.0/docs/Data-String.html#t:String)? Bear in mind that I cannot modify the import statement.
Thank you.
To make links to external packages in the Haddock documentation, you need to instruct it where to find the documentation for those packages.
It is done by using the --read-interface Haddock command-line option.
Using your example, it will be :
haddock --read-interface module-A-package/docs/,module-A-package/docs/module-A-package.haddock
The .haddock file is made when generating documentation for the package module-A-package using ----dump-interface Haddock command-line option.
More information can be found on the Haddock documentation or this HaskellWiki page.
I have 3 data constructors say A,B and C that are defined in files A.hs, B.hs, C.hs and the files are in the directory project-utils.
Now I want to use these data constructors in parts of other projects. These projects reside in totally different directories.
How do I import the data and type constructors of A, B and C in such project files?
Thanks to the first answer given below, I realized that I am looking for a skeleton to organize such project in a better way. I searched but could not find any such project skeleton.
The link provided there contains many things that are described in vague manner. For example, on line 5 there it is simple written as "..."
What i am looking for is,
The skeleton project should not be very simple "single" file project as is given on the Haskell site. But should NOT be overly complex with tons of dependencies etc that we see in many projects on hackage.
Edit: I changed the title to reflect my problem in a better way. Sorry for the inconvenience.
Make a cabal pkg out of them and install that package locally.
Follow a directory stucture as here and use those constuctors in a project rather than across projects. The stucture mentioned is basically a structure of a cabal package.
Manually add the input source while compiling through ghc or loading in ghci.
Example
ghci -i project-utils/A.hs Foo.hs
where Foo.hs uses elements exported by A.hs
May not be exactly what you're looking for, but for future readers of this question, a Haskell skeleton/template project was just released here:
https://github.com/tfausak/haskeleton
It does add some dependencies like hlint and hspec. Here is the blog post, which goes through each of the individual decisions:
http://taylor.fausak.me/2014/03/04/haskeleton-a-haskell-project-skeleton/
I found this: how to write a haskell program link as a handy reference.
#Tem Pora : you need to install yesod and yesod-bin. This link talks more about the scaffolding
cabal install yesod
cabal install yesod-bin
<cdtoprojdir> yesod init
Hope this helps.
I'm going to guess the answer is 'no', but is there a way to import a symbol from a module that is not explicitly export from said module?
I tried looking up various ghc (6.12.1) flags like -XPackageImports, but I don't see anything that will do what I want? (which is purely for doing some quick & dirty testing without re-compiling some other modules; i don't actually want to write code this way)
No. There isn't even a dirty hack. The .hi files which are used by the type checker to find out the types of things of imported modules only contain info about exported names. The only way to change that is to edit the source file.
No