In haskell build system, Cabal as well as runhaskell has got almost same set of sub-commands, configure, build. For runhaskell, it is:
runhaskell Setup.hs configure
runhaskell Setup.hs build
...whereas for cabal it is:
cabal build
cabal configure
So, when should I use which command? Do both the commands have same functionality?
Can I run runhaskell without sudo access, because I see runhaskell makes entries inside /opt/ghc/7.8.4/lib whereas cabal works in sudo/non-sudo mode?
cabal and runhaskell serve entirely different purpose.
runhaskell is used for executing Haskell programs without
having the need to compile them. You can place this on the top of an Haskell file: #!/usr/bin/env runhaskell and give it the scripting ability. The cabal is a package manager and a build system
for Haskell.
Also cabal and runhaskell don't share their sub-commands like
configure, build, install etc.
i saw in this link bob.ippoli.to/archives/2013/01/11/getting-started-with-haskell, two commands being used... runhaskell Setup.hs configure && runhaskell Setup.hs build
Usually Setup.hs has the following code when generated from cabal init:
import Distribution.Simple
main = defaultMain
Now if you see the implementation of defaultMain:
defaultMain :: IO ()
defaultMain = getArgs >>= defaultMainHelper simpleUserHooks
So what you are passing is actually command line arguments which can be anything. In fact you can test that yourself:
$ runhaskell Setup.hs invalid_argument
unrecognised command: invalid_argument (try --help)
runhaskell doesn’t have any subcommands:
$ runhaskell --help
Usage: runghc [runghc flags] [GHC flags] module [program args]
It takes a path to a Haskell source file and executes it right away. So you should use it when all you want is to run some Haskell code you have. Of course, you can use runhaskell without superuser access; I doubt it creates anything in /opt.
Cabal is a build-system for Haskell projects, its purpose is transforming Haskell sources into binaries.
Related
I'm relatively new to Haskell and I realize I might be swimming against the stream here, but nonetheless, I'll ask:
Say I have a short Haskell script:
import Data.List.Split (splitOn)
main :: IO ()
main = do
let orders = splitOn "x" "axbxc"
putStrLn $ head orders
If I used only standard functions I could compile this with ghc <script.hs>. Because I depend on the split package to provide the splitOn function, the compilation fails.
Now, I have no difficulties setting up a cabal project with a project.cabal and a Setup.hs file in order to get this to actually compile. However, this feels like a lot of extra boilerplate for a standalone script.
So, is there a way to compile a single .hs file against some external package? Something similar to what in Python would be done by pip install something, "installing the package into the interpreter", i.e. is there a way to install extra packages "into ghc", so that I for instance only need to provide some extra linking flag to ghc?
The Cabal equivalent of the Stack script in bradrn's answer would be:
#!/usr/bin/env cabal
{- cabal:
build-depends: base
, split
-}
import Data.List.Split (splitOn)
main :: IO ()
main = do
let orders = splitOn "x" "axbxc"
putStrLn $ head orders
The script can be run with cabal run, or directly by giving it execute permission. If need be, version bounds can be added as usual to the build-depends on the top of the script.
(Note this isn't literally a solution without Cabal, as doing this with GHC alone, even if it is possible, wouldn't be worth the trouble. In any case, it certainly avoid the boilerplate of needing multiple files.)
If you use Stack, the simplest way to do this is to write a ‘Stack script’, which is a Haskell file with a description of the required packages in the first line (really an invocation of stack specifying the appropriate command line arguments). An example (slightly modified from the docs):
$ cat turtle-example.hs
-- stack --resolver lts-6.25 script --package turtle
{-# LANGUAGE OverloadedStrings #-}
import Turtle
main = echo "Hello World!"
$ stack ./turtle-example.hs
Completed 5 action(s).
Hello World!
$ stack ./turtle-example.hs
Hello World!
This script uses the turtle package; when run, Stack downloads and builds this dependency, after which it is available in the script. (Note that the second time it is run, turtle has already been built so does not need to be rebuilt again.)
As it happens, the --package command in Stack is not limited to scripts. It can be used with other Stack commands as well! For instance, to compile your program, you should be able to run stack ghc --resolver lts-16.27 --package split -- -ghc-options your-program-name.hs. And stack ghci --package split will give you a GHCi prompt where you can import Data.List.Split.
(Note: This answer focuses on Stack rather than Cabal, simply because I don’t know Cabal very well. However, I believe all this can be done using Cabal as well. For instance, I do know that Cabal has something very similar to the Stack scripts I mentioned above, though I can’t remember the syntax just at the moment.)
EDIT: See #duplode’s answer for how to do this with Cabal.
You can install into the default environment for the current user by doing cabal install --lib split. The package should then be available to ghc and ghci without needing any special options.
More information is at the bottom of this section in the Cabal manual. The v2 commands that it uses are the default now so if you have a fairly new cabal you can just use install rather than v2-install.
I think this is the quintessential entry point to the package management battle in Haskell. It's not there's not enough advice, but there's so much, each with its own caveats and assumptions. Climbing that mountain for the sake of splitOn feels to the newbie like they're Doing It Wrong.
After spending far too much time trying each permutation, I've collated the fine answers here, and many, many others from elsewhere, put them to the test, and summarised the results. The full write up is here.
The pertinent summary of solutions is:
Install globally
You can still do global installs with cabal install --lib the-package.
Use Stack as a run command
You can use stack directly, eg: stack exec --package containers --package optparse-generic [...and so on] -- runghc hello.hs
Create a Stack project
The real deal. Run stack new my-project hraftery/minimal, put your code in Main.hs and your dependencies in my-project.cabal (and maybe stack.yaml - check the article), and then run stack build to automagically pull and build all dependencies.
Use a Stack script
Make your Haskell file itself an executable Stack script. Add -- stack --resolver lts-6.25 script --package the-package to the top of your .hs file and set its executable bit.
For my Edit/Test/Run workflow (eg. using VS Code, GHCi and GHC respectively), I found it pretty clear what works in practice. In summary:
Amongst a chorus of discouragement, Global Installs suit what I know of your use case just fine.
Where Global Installs don't make sense (eg. for managing dependency versions or being portable) a Stack project starting from my minimal template is a smooth transition to a more sophisticated and popular method.
I'm getting an error that says the module doesn't exist when I try to runhaskell. It's odd because I try to install it first and says its up to date. Any idea how to fix this?
You could try creating the package environment in the local directory that holds your project, like this:
c:\Users\...\ex1haskell> cabal install --lib --package-env . QuickCheck
This should create a file of the form .ghc.environment.xxx in ex1haskell, which hopefully should be picked up by runhaskell/ghci/ghc invocations.
In ghci sessions, a sign that the environment is being picked up is the following message while starting:
Loaded package environment from ...
When the --package-env location is not given explicitly, a default location is used. According to the docs:
By default, it is writing to the global environment in
~/.ghc/$ARCH-$OS-$GHCVER/environments/default. v2-install provides the
--package-env flag to control which of these environments is modified.
But it seems that runhaskell is having problems to find the environment file in that default location.
Note. When creating a package environment, it's possible to specify multiple packages simultaneously, like this:
cabal install --lib --package-env . QuickCheck random aeson transformers
Also, package environments are just text files, so local environments can be deleted and recreated at will. The actual package binaries reside elsewhere and can potentially be reused by cabal.
A Common Environment
It is hard to debug if/when the actual tooling differs so let's first get a unified setup. I'll use docker to get GHC 8 and Cabal 3.x:
docker run --rm -it haskell bash
Understand that this isn't arbitrary or even preemptive. What you have shown - cabal install --lib ... and runhaskell ... does work for sane tool installations. You might have a bad installation or an old version of a tool that has different behavior.
Running a single file with runhaskell
Now we need a project:
root#8a934c302dba:/# mkdir Ex1
root#8a934c302dba:/# cd Ex1
root#8a934c302dba:/Ex1# cat <<EOF >Main.hs
> import Test.QuickCheck
>
> main :: IO ()
> main = print =<< (generate arbitrary :: IO Int)
> EOF
And test failure:
root#8a934c302dba:/Ex1# runhaskell Main.hs
Main.hs:1:1: error:
Could not find module `Test.QuickCheck'
Use -v (or `:set -v` in ghci) to see a list of the files searched for.
|
1 | import Test.QuickCheck
And install the library:
root#8a934c302dba:/Ex1# cabal update && cabal install --lib QuickCheck
And successful run:
root#8a934c302dba:/Ex1# runhaskell Main.hs
15
So my comment above was wrong - we don't need to explicitly list the package as it is already exposed after installation.
runghc -package-db=.cabal-sandbox/.cabal-sandbox/x86_64-osx-ghc-7.8.3-packages.conf.d hellowai.hs
Works perfect for me.
Similarly, with
ghci -package-db=.cabal-sandbox/.cabal-sandbox/x86_64-osx-ghc-7.8.3-packages.conf.d
I am also able to import my cabal-sandbox-installed Wai package in ghci with no issue at all.
But when I ask ghc-mod to validate my haskell source code, via
ghc-mod check --boundary="" -g -package-db=.cabal-sandbox/x86_64-osx-ghc-7.8.3-packages.conf.d hellowai.hs
hellowai.hs:4:8:Could not find module ‘Network.Wai.Handler.Warp’Use -v to see a list of the files searched for.
hellowai.hs:3:8:Could not find module ‘Network.HTTP.Types’Perhaps you meant Network.HTTP.Base (from HTTP-4000.2.19) Network.HTTP.Base (needs flag -package HTTP-4000.2.10) Network.HTTP.Headers (needs flag -package HTTP-4000.2.10)Use -v to see a list of the files searched for.
hellowai.hs:2:8:Could not find module ‘Network.Wai’Perhaps you meant Network.BSD (needs flag -package network-2.4.2.3) Network.URI (needs flag -package network-2.4.2.3) Network.TCP (needs flag -package HTTP-4000.2.10)Use -v to see a list of the files searched for.
It is unable to find my cabal sandbox installed module. Why is that so?
Do you have a cabal.sandbox.config file? And are you using a .cabal file for your project?
If you have both of these you should be able to use ghc-mod check ... and it will just work.
Another advantage of using a .cabal file is that you can use cabal repl to invoke ghci and cabal run to invoke runhaskell with the correct command line options.
Update
Here is a recipe you can try out to see when ghc-mod can find your cabal sandbox. Perhaps this can help you determine what's different with your set up.
Start in a clean directory:
$ mkdir foo
$ cd foo
$ cabal sandbox init
$ cabal get split
$ cd split-0.2.2
$ cabal sandbox init --sandbox=../.cabal-sandbox
Edit around line 55 of split.cabal to add heredoc as a dependency.
Edit src/Data/List/Split.hs to use the module Text.Heredoc:
{-# LANGUAGE QuasiQuotes #-}
...
import Text.Heredoc
...
foo :: String
foo = [here|this is a test|]
Make sure heredoc is installed:
$ cabal install --only-dependencies
Finally this should work:
$ ghc-mod check ./src/Data/List/Split.hs
And it will still work if you cd into a sub-directory:
$ cd src
$ ghc-mod check ./Data/List/Split.hs
However, ghc-mod won't work if you move away split.cabal:
(back at the top level directory)
$ mv split.cabal split.cabal-old
$ ghc-mod check ./src/Data/List/Split.hs
In this case I created the sandbox in a parent directory of our working directory, but things should also work if the initial sandbox was created like this:
$ mkdir foo
$ cd foo
$ mkdir sandbox-dir
$ cd sandbox-dir
$ cabal sandbox init
$ cd ..
$ cabal get split
$ cd split-0.2.2
$ cabal sandbox init --sandbox=../sandbox-dir/.cabal-sandbox
I’m trying to write a project with the Hakyll library. In order to avoid messing up with my system, I’d install it in a cabal sandbox in the same folder where my Hakyll project lives.
Being more or less a beginner, I’m still struggling with getting the pest practices right. A simple approach would be to just do
$ cabal sandbox init
$ cabal install hakyll
$ cabal exec ghc -- --make site.hs
where the last line compiles my Hakyll generator using the libraries in the sandbox. The obvious disadvantage is that this is not reproducible. The main version of Hakyll could have change when I try to run it again from a clean checkout.
Another approach would be to write a proper project.cabal file (For example like this: chromaticleaves.cabal) and then do cabal install or cabal run.
However, I feel that this may be a bit too much information. As I do not intend to publish this project any more than needed, I’m not really convinced I need to put a project name and version number in there. (For example, in a Ruby Gemfile, I would also only specify the libraries and nothing else unless I wanted to publish a gem myself.)
So, eventually I figured that with a file like
$ cat project.cabal
cabal-version: >= 1.2
library
build-depends: base >=4.6
, containers
, process
, hakyll >=4.5
, pandoc
, pandoc-types
I can type
$ cabal sandbox init
$ cabal install --only-dependencies
$ cabal exec ghc -- --make site.hs
and it seems to download all dependencies and is able to compile the file.
Is this a reasonable approach or is the best practice really to give a full specification with name, version and executable sections in the cabal file?
Edit: Apparently, my approach does not let me do cabal repl. So either there exists a completely different way of doing it or it seems I have to go with a fuller specification.
I use your first approach myself for my Hakyll-based webpage. You don't need to create a .cabal file to pin the Hakyll version, you only need to add the following line to cabal.config:
constraints: hakyll == 4.5
I think that cabal repl will work with this approach, but you will need to load site.hs manually (:l site.hs). Or you can use cabal exec ghci -- site.hs.
cabal sandbox init
cabal install haskeline
... installs successfully ...
ghci
Prelude> :module +System.Console.Haskeline
<no location info>:
Could not find module `System.Console.Haskeline'
ghc-pkg list haskeline
.. not found ..
What do I have to do get haskeline to work with cabal sandbox? If I install haskeline normally (no sandbox) it is fine (ghc-pkg list haskeline -- found it).
Either use cabal repl like Joseph mentioned or you can explicitly pass the package db to the GHCi shell relative to your current working directory.
ghci -no-user-package-db -package-db .cabal-sandbox/*-packages.conf.d YourModule.hs
It's recommended that you just use cabal.
In order to get ghci to use a local sandbox you must (a) set up a my-project.cabal file and (b) use cabal repl.