Once you've installed Stack, you can use it to install GHC for you. Great!
...now how do I compile a file with it?
To be clear: What you're supposed to do is write a package specification and have Stack build that. But surely there must be a way to trivially compile a tiny helper file inside that project? It seems silly to have to construct an entire project just so I can build a tiny helper program and have it easily runnable from the shell.
I know I can run any Haskell file using Stack. But I cannot for the life of me figure out how to compile the thing...
You can use stack ghc -- <file names> to compile and link a set of files (it's briefly mentioned in Stack's user guide):
You'll sometimes want to just compile (or run) a single Haskell source file, instead of creating an entire Cabal package for it. You can use stack exec ghc or stack exec runghc for that. As simple helpers, we also provide the stack ghc and stack runghc commands, for these common cases.
The -- is to ensure the arguments we pass are sent to ghc, rather than being parsed as arguments to stack exec. It's the same thing as when trying to pass arguments to an executable you've made using the normal stack toolchain: stack exec myExe -foo passes -foo to exec, not myExe, stack exec myExe -- -foo behaves as desired.
For example:
Bar.hs
module Bar where
bar :: Int
bar = 5
Foo.hs
import Bar
main :: IO ()
main = print bar
Compilation (don't even need to specify Bar.hs in the build files, it's sourced automatically):
> stack ghc -- Foo.hs
[1 of 2] Compiling Bar ( Bar.hs, Bar.o )
[2 of 2] Compiling Main ( Foo.hs, Foo.o )
Linking Foo ...
> ./Foo
5
There's no problem with dependencies either - it looks like all the packages installed locally are available to the build (you can use containers or QuickCheck without any additional build params).
Related
I am trying to learn how to structure a Haskell project/workflow that uses FFI.
I am using stack, but I find myself unable to use GHCi when it comes to the imported foreign functions.
Here is a simplified version of the problem. Let's say that I have the following two files in $PROJECT_ROOT/cbits:
hello.h
#ifndef HELLO_H
#define HELLO_H
extern "C"
{
int foo();
}
#endif /* HELLO_H */
hello.cpp
#include "hello.h"
#include <iostream>
int foo()
{
std::cout << "extremely dangerous side effect" << std::endl;
return 42;
}
My Main.hs file:
module Main where
import Foreign.C
foreign import ccall unsafe "foo" foo :: IO CInt
-- this does side effects and prints '42'
main = foo >>= print
The relevant (C++ specific) section of my package.yaml is:
include-dirs:
- cbits
cxx-sources:
- cbits/*.cpp
cxx-options:
- -std=c++17
extra-libraries:
- stdc++
I am using the souffle-haskell's package.yaml as a reference.
Compiling and running with stack run is ok and I get the expected output:
extremely dangerous side effect
42
But, in the GHCi session (run with stack ghci), calling main gives:
ghc: ^^ Could not load 'foo', dependency unresolved. See top entry above.
GHC.ByteCode.Linker: can't find label
During interactive linking, GHCi couldn't find the following symbol:
foo
This may be due to you not asking GHCi to load extra object files,
archives or DLLs needed by your current session. Restart GHCi, specifying
the missing library using the -L/path/to/object/dir and -lmissinglibname
flags, or simply by naming the relevant files on the GHCi command line.
Alternatively, this link failure might indicate a bug in GHCi.
If you suspect the latter, please report this as a GHC bug:
https://www.haskell.org/ghc/reportabug
The problem is not present if I compile hello.cpp beforehand:
g++ -c cbits/hello.cpp -o cbits/hello.o
And then run stack ghci --ghci-options cbits/hello.o, as suggested by the GHCi error message.
Question is: do I really need to maintain a separate *.o file specifically for GHCi? Searching online I have found discussions addressing only the GHCi part or the stack/cabal part, but not both. The only useful answer that I have found is this one from 2013, which reaffirms the "solution" given by GHCi and does not mention stack or cabal.
Question is: do I really need to maintain a separate *.o file specifically for GHCi?
Answer is: no.
After several tries, the only thing that I had to change was the name of an option:
- cxx-sources:
+ c-sources:
This left the behaviour of stack run unchanged, and allowed GHCi to link properly to the compiled code.
I have a small haskell program which builds and executes with stack ok. When I start it with stack ghci I have an error message which I do not understand and cannot proceed.
GHCi, version 8.10.4: https://www.haskell.org/ghc/ :? for help
[1 of 3] Compiling Lib ( /home/frank/Workspace11/primo/src/Lib.hs, interpreted )
[2 of 3] Compiling YamlRead ( /home/frank/Workspace11/primo/src/YamlRead.hs, interpreted )
[3 of 3] Compiling Main ( /home/frank/Workspace11/primo/app/Main.hs, interpreted )
Ok, three modules loaded.
Loaded GHCi configuration from /home/frank/Workspace11/primo/.ghci
<no location info>: error:
module ‘main:Main’ is defined in multiple files: /home/frank/Workspace11/primo/app/Main.hs
/home/frank/Workspace11/primo/app/Main.hs
I do not see why the same Main is listed twice in the exact same file.
I had a somewhat similar warning message about Paths_primo which is a known bug ( Stack issue #5439 ) and I fixed following the advice see.
What is the cure against this error? I have not used stack much - am I doing something wrong?
This looks like a sign that Main.hs or Main is inadvertently listed multiple times in your Stack package.yaml, in such a way that ghc is invoked with multiple occurrences of it.
This error is possible to reproduce easily with GHC alone, for example:
> echo 'main = putStrLn "hello"' > Hello.hs
> ghc Hello Hello.hs
<no location info>: error:
module ‘main:Main’ is defined in multiple files: Hello.hs Hello.hs
I would run Stack with --verbose and see how GHCi is being invoked, and double-check the package.yaml and generated Cabal file. (If you edit your question to include that, we may be able to offer more specific help with it.)
I can think of several possible reasons for this, such as literally listing Main or Main.hs multiple times (e.g. in exposed-modules, other-modules, main-is); or an interaction like a missing option value in the ghc-options field causing subsequent flags to be misinterpreted.
Main.hs can be listed multiple times in GHCi invocation when it is present in multiple components. Typical way when this happens is:
You create a project with stack new which contains single src/Main.hs file, so package.yaml has an executable entry and no library entry.
Then you add a library and use source-dir: src. Now Main.hs is listed in two components: myapp-lib and myapp-lib-exe. Assuming you package name is myapp.
To solve the problem you should move Main.hs to app dir and update executables:source-dirs to app in package.yaml.
So the problem may happen when Main.hs is listed in multiple components in package.yaml, for example in executables and library.
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.
A minimal reproduction can be found here:
https://github.com/IvanMalison/stack-gtk2hs-bug
Everything works as expected when I use normal stack commands, but when I run the failing command:
stack ghc -- --make main.hs
I get the following error:
main.hs:3:1: error:
Ambiguous interface for ‘Graphics.UI.Gtk’:
it was found in multiple packages: gtk-0.14.6 gtk3-0.14.6
main.hs:4:1: error:
Ambiguous interface for ‘Graphics.UI.Gtk.Abstract.Widget’:
it was found in multiple packages: gtk-0.14.6 gtk3-0.14.6
main.hs:5:1: error:
Ambiguous interface for ‘Graphics.UI.Gtk.Layout.Table’:
it was found in multiple packages: gtk-0.14.6 gtk3-0.14.6
The output of stack exec ghc-pkg -- --no-user-package-db list is https://gist.github.com/f19f900988f49e4d03cd61f1cab48baa . This output makes me expect that the reason that this is happening is that some other stack install required gtk (not gtk3 which is what is specified as a dependency in this package) and somehow this package is visible from the stack ghc command for some reason.
Am I misunderstanding the stack ghc command? Shouldn't this essentially do the same thing as stack build?
There's no builtin way to do this with stack currently. However, it is possible to get stack ghci to do this. The most straightforward way to do it is to make a cabal package which has the executable target. However, if you really want to just use straight ghc, there is a way. Copy-pasting from my comment here:
stack ghc works a bit differently than stack ghci. It's essentially a synonym for stack exec -- ghc, which will run the right compiler with the right databases, but won't set up anything related to your local packages like include directories etc. Note that stack ghci takes TARGET arguments whereas stack ghc does not. Retrospectively, this is a bit inconsistent, but stack ghc came before stack ghci.
It does make sense to have the ability to do something like this, though not sure how to best achieve that. Some potential options:
--no-interactive argument on stack ghci. Would be a bit obtuse. Weird to run a ghci command when, though it would be using the stack ghci logic.
Add --target TARGET option to stack ghc, to tell it to use the environment of a particular local package target.
Here's a workaround for now. Put the following in ~/.local/bin/stack-run-ghc.sh and make it user executable:
#/bin/sh
ghc $(echo "$*" | sed 's/--interactive//g')
This takes the arguments, removes --interactive, and calls ghc. With this, I can build stack using ghc via the following:
stack ghci --with-ghc stack-run-ghc.sh --ghci-options src/main/Main.hs
I have following problem with ghc-mod which prevents me from using ide for some files in a yesod app project.
I install template app as follows:
/tmp$ stack new demo yesod-sqlite && cd demo
/tmp/demo$ stack setup && stack build && stack install ghc-mod
Which yields following stack.yaml (commented lines removed):
resolver: lts-5.6
packages:
- '.'
extra-deps: []
flags: {}
extra-package-dbs: []
And this is a demo.cabal: http://pastebin.com/i4n1TR6W.
Then, running stack exec -- ghc-mod check app/main.hs does not produce errors, but stack exec -- ghc-mod check app/devel.hs has this to say:
app/devel.hs:2:1:Failed to load interface for ‘Application’It is a member of the hidden package ‘demo-0.0.0’.Perhaps you need to add ‘demo’ to the build-depends in your .cabal file.
So the ghc-mod somehow thinks this package is itself hidden? But any other place where project's files are imported by another checks fine, and the application builds and works successfully. The only specifics about this file is using PackageImports language extension:
{-# LANGUAGE PackageImports #-}
import "demo" Application (develMain)
I tried googling the error message but it seems to only come up with regard to external packages and not the one being debugged.
These two files devel.hs and DevelMain.hs are quite special: they are marked as a module of demo in .cabal but they are importing demo as a compiled package, i.e. recursive dependency.
They are not exposed from library demo nor imported anywhere else so won't get compiled when you run stack build, but when you run ghc-mod check on them, they are interpreted in the context of the current project, therefore the recursive dependency will be an issue.
The only purpose of these two otherwise meaningless files is to debug your yesod website in ghci, as the comment in DevelMain.hs stated:
-- | Running your app inside GHCi.
--
-- To start up GHCi for usage with Yesod, first make sure you are in dev mode:
--
-- > cabal configure -fdev
--
-- Note that #yesod devel# automatically sets the dev flag.
-- Now launch the repl:
--
-- > cabal repl --ghc-options="-O0 -fobject-code"
--
-- To start your app, run:
--
-- > :l DevelMain
-- > DevelMain.update
--
-- You can also call #DevelMain.shutdown# to stop the app
--
-- You will need to add the foreign-store package to your .cabal file.
-- It is very light-weight.
--
-- If you don't use cabal repl, you will need
-- to run the following in GHCi or to add it to
-- your .ghci file.
--
-- :set -DDEVELOPMENT
--
-- There is more information about this approach,
-- on the wiki: https://github.com/yesodweb/yesod/wiki/ghci
cabal repl and stack ghci will compile the project beforehand so these two files won't cause any error there.