I'm trying to run some Safe Haskell code with runghc, but it doesn't seem to work for me.
bgeron#tinker:/tmp/wtf$ ls
Strange.hs
bgeron#tinker:/tmp/wtf$ cat Strange.hs
module Strange where
main :: IO ()
main = do
return ()
bgeron#tinker:/tmp/wtf$ runghc -XSafe Strange
Strange:1:1: Not in scope: `System.Environment.withArgs'
bgeron#tinker:/tmp/wtf$ runghc --version
runghc 7.6.3
I thought this would be a valid use of runghc; the error is most confusing. Is this a bug?
I'm using Ubuntu 14.04, 64-bit.
The observed behaviour can be explained by the following.
The implementation of runghc is here:
https://ghc.haskell.org/trac/ghc/browser/ghc/utils/runghc/runghc.hs
It will make the following call to ghc, which also shows the strange behaviour:
ghc -XSafe -e ':main' Strange.hs
The implementation of ghc in evalatue-expression mode will add the offending import:
https://ghc.haskell.org/trac/ghc/browser/ghc/ghc/InteractiveUI.hs#L1154
I am not sure whether it's a bug. I agree it's confusing.
As mentioned in the comment, just add import System.Environment
Related
I have installed Haskell platform following the instructions on chocolatey and haskell.org.
I am using Windows 10.
My hello.hs program complies in command prompt, but when I try to do the same in VS code,
it won't load Prelude, which I assume is necessary for running Haskell programs.
I think it might be a configuration problem, but I cant find any useful documentation on it.
How could I fix this and turn on Prelude?
Are the problems that VS code shows relevant to this problem?
Nothing looks wrong in your screenshots. The prompt text is ghci> rather than Prelude> because GHCi no longer defaults to showing the loaded modules on the prompt (see the GHC 9.0.1 changelog. Prelude is being loaded regardless. The warning shown in the IDE is a style suggestion that is inconsequential for the purpose of getting your code to run. As chi suggests, :l hello.hs at the GHCi prompt should be enough to have it loaded.
If I type main into the console, then it works
ghci> main
"haskell"
name
"hi, name"
ghci>
This might work
$ ghci
ghci> import Prelude
Prelude> printStrLn "Hello World!"
Hello World!
Prelude>
You may need to just run import Prelude.
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 am learning yesod and would like to know, which code is generated behind, when I am using
mkYesod "HelloWorld" [parseRoutes|
/ HomeR GET
|]
function.
In the doc, it says:
We’ll look at this in more detail in the routing chapter. But by using
the -ddump-splices GHC option, we can get an immediate look at the
generated code.
How can I pass -ddump-splices to GHC option?
I start the application with:
stack runghc -ddump-splices helloworld.hs
I think you can use the OPTIONS_GHC pragma:
simply add
{-# OPTIONS_GHC -ddump-splices #-}
at the top of your file.
I don't see a way to pass options through to GHC when using stack runghc. You can compile your project with stack build, which accepts more options. The full command is stack build --ghc-options '-ddump-splices'.
After compiling, you may also want to run your project. If you followed the Yesod quickstart, I think stack exec -- yesod devel will work.
I ran across this today when working through the Yesod Book. The following seems to work well. Any option passed before -- is passed to stack and options after -- are passed directly to GHC
stack runghc -- -ddump-splices helloworld.hs
My ghci version is 8.4.3
I tried
stack install QuickCheck
Something was installed. But when I input import Test.QuickCheck, it tells Could not find module ‘Test.QuickCheck’ again. How can I fix it?
Firstly, stack install is not recommended for installing executables or libraries. Instead, there's a couple of things you can do to use the QuickCheck library:
If you want to use QuickCheck in a command such as stack ghci or stack ghc, you can add it as a --package option e.g. to run a REPL to play around with QuickCheck you can use stack ghci --package QuickCheck and then write import Test.QuickCheck.
If you want to write a small one-file program using QuickCheck, then you can run stack ghc --package QuickCheck -- MyProgram.hs (using the --package option from the last bullet point). Alternately, you can use stack's scripting functionality and include a line such as this at the top of your program:
-- stack --resolver lts-12.18 script --package QuickCheck
If you want to use QuickCheck in a large project, then add it as a dependency in your my-program.cabal or project.yaml file.
The same guidance applies to any package you may want to use.
myos>cabal update
myos>cabal install --lib QuickCheck
myos>ghci
gchi> import Test.QuickCheck
If I have the following in my package.yaml file:
default-extensions:
- LambdaCase
I am able to compile my project which makes use of the LambdaCase syntax like this:
myFunction = \case
Nothing -> "empty"
Just x -> x
However, if the project is run with stack runghc, the LambdaCase extension is not respected.
My project has about 200 modules, so I would rather not have to add {-# LANGUAGE LambdaCase #-} to the top of every file.
Is there a way to enable a project-wide GHC extension with stack runghc analogously to the package-wide default-extensions property in package.yaml?
Yes, stack should probably have some better support for this -
see https://github.com/commercialhaskell/stack/issues/3338 .
I'd say the summary is that stack runghc came before stack ghci, it's ended up having a much simpler meaning that does not take cabal files into account at all. Not sure how to make things consistent and intuitive on the commandline without changing the meaning of runghc.
In that issue, I describe a hacky workaround. Copying it here:
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