I put this in ~/Desktop/Shapes.hs:
module Shapes
( Shape(Rectangle)
) where
data Shape = Circle | Rectangle deriving (Show)
I then do this:
cd ~/Desktop
ghci
ghci> :m +Shapes
<no location info>:
Could not find module `Shapes'
It is not a module in the current program, or in any known package.
ghci> import Shapes
<no location info>:
Could not find module `Shapes'
It is not a module in the current program, or in any known package.
Why do I get this error?
I also tried compiling first with ghc -c Shapes.hs. It still doesn't work.
I installed the "Haskell Platform 2013.2.0.0 for Mac OS X, 64 bit" from haskell.org on my OS X 10.9.2 Mavericks. I also followed their ghc-clang-wrapper instructions.
Update:
Someone suggested doing :l Shapes.hs first. The problem is that :l Shapes.hs loads the entire Shapes file, meaning I have access to the Circle value constructor even if I did not export it. See my earlier question: Why can I use this "private" value constructor? I want to load only the module. Is this possible?
You need to load your Shapes.hs first by :l Shapes.hs.
Because your Shapes have not been loaded, so :m Shapes will not work.
Because your Shapes does not exist in a compiled package that ghci can find, so import Shapes will not work.
If you only want the exported symbols in scope, after :load a module, you could use :module or import to only import those symbols. For example, after :load Shapes.hs and :module Shapes, Rectangle will in scope but Circle will not.
See:
What's really in scope at the prompt?
:module and :load
Related
I have a haskell package called prime-tools. When I use stack ghci inside the package directory, I expect it to open up an interactive ghci and automatically import prime-tools. Instead, I find that it loads all of the modules declared in the .cabal file.
For example, here is the extract from my .cabal file showing which modules are declared:
library
-- Modules exported by the library.
exposed-modules: PrimeTools.MathStuff, PrimeTools.Factors, PrimeTools.PQTrials, PrimeTools.Main, PrimeTools.Base, PrimeTools.Lucas
-- Modules included in this library but not exported.
other-modules: PrimeTools.Extras
And this is what happens as I receive the ghci> prompt after running stack ghci in the project folder:
Ok, modules loaded: PrimeTools.MathStuff, PrimeTools.Factors, PrimeTools.PQTrials, PrimeTools.Main, PrimeTools.Base, PrimeTools.Lucas, PrimeTools.Extras.
ghci>
The problem with loading the modules instead of import prime-tools is that I can now use all functions defined in all modules whether or not they are exported.
An example of the issues arising from this distinction: there are two modules in package prime-tools which have implementations of a function called pfactor. One of them is exported, and intended to be used by the end user of the package, while the other is for internal use only and not exported.
Before someone comments, there is good reason to have two implementations of pfactor, but it's not relevant to my question.
My question: how can I use stack to automatically start up a ghci environment with the local version of ghc and import the package whose folder I run the command in?
My desired behaviour would be equivalent to running the following sequence of commands:
stack ghci
ghci> :module -PrimeTools.MathStuff
ghci> :module -PrimeTools.Factors
ghci> :module -PrimeTools.PQTrials
ghci> :module -PrimeTools.Main
ghci> :module -PrimeTools.Base
ghci> :module -PrimeTools.Lucas
ghci> :module -PrimeTools.Extras
ghci> import PrimeTools.MathStuff
ghci> import PrimeTools.Factors
ghci> import PrimeTools.PQTrials
ghci> import PrimeTools.Main
ghci> import PrimeTools.Base
ghci> import PrimeTools.Lucas
The key here is that I want to import the exposed-modules declared in the .cabal file and not to load any modules. I don't mind if the other-modules are also imported, though. Is there a way I can do this using stack without having to run this long sequence of commands every time?
A reasonable workaround is defining a custom GHCi macro for importing your modules the way you want it. Create a .ghci file in your project root along these lines:
:{
:def deload (\_ -> return
":module -PrimeTools.MathStuff\n\
\import PrimeTools.MathStuff"
)
:}
With this, the :deload command in GHCi will remove from scope and then re-import PrimeTools.MathStuff -- you can add as many modules as you want to the list. Though I have written this using multiline string syntax, you can have any String -> IO String expression within the parentheses, so feel free to spell or extend it however you see fit.
I encounter a strange situation in GHCI, don't know if anyone observed similar case. For some module, when I have it in the search path both by -package-conf, and also by -i, GHCI fails when I try to import the module with 'module is not loaded: FooModule'.
:module loads it fine however
or I can do :load FooModule, :m to clear the import list, and then import FooModule
or I can remove the path from -i and then it imports fine
Tracked this to be the otherwise -> modNotLoadedError m loc case in GHC, where otherwise ~ modulePackageId = this_pkg (the meaning of which I don't know).
This is not entirely systematic, there are some module which are both in package and source path, but can be imported.
GHC only knows about packages that are installed. To see which packages are installed, use the ghc-pkg list command:
$ ghc-pkg list
/usr/lib/ghc-6.12.1/package.conf.d:
Cabal-1.7.4
array-0.2.0.1
base-3.0.3.0
base-4.2.0.0
bin-package-db-0.0.0.0
binary-0.5.0.1
bytestring-0.9.1.4
containers-0.2.0.1
directory-1.0.0.2
(dph-base-0.4.0)
(dph-par-0.4.0)
(dph-prim-interface-0.4.0)
(dph-prim-par-0.4.0)
(dph-prim-seq-0.4.0)
(dph-seq-0.4.0)
extensible-exceptions-0.1.1.0
ffi-1.0
filepath-1.1.0.1
(ghc-6.12.1)
ghc-prim-0.1.0.0
haskeline-0.6.2
haskell98-1.0.1.0
hpc-0.5.0.2
integer-gmp-0.1.0.0
mtl-1.1.0.2
old-locale-1.0.0.1
old-time-1.0.0.1
pretty-1.0.1.0
process-1.0.1.1
random-1.0.0.1
rts-1.0
syb-0.1.0.0
template-haskell-2.4.0.0
terminfo-0.3.1
time-1.1.4
unix-2.3.1.0
utf8-string-0.3.4
In haskell interactive mode, ghci.
If I have imported one package, such as: import Data.List
I went to Prelude Data.List>
My question is, how to get back to "Prelude"
Thanks,
GHCi "meta" commands begin with :. One of them, :modules (:m for short) lets you modify the currently loaded modules. :m by itself will reset all of the loaded modules returning you to Prelude>.
You can also use :m +MODULE to "add" the module MODULE or :m -MODULE to remove the module MODULE.
Is there a way to specify the package name for a module for the :browse, :load or :module commands in ghci (version 6.12.1) ?
Some module names are ambiguous:
Prelude> :module Control.Monad.Cont
<no location info>:
Ambiguous module name `Control.Monad.Cont':
it was found in multiple packages: mtl-1.1.0.2 monads-fd-0.1.0.2
Is setting the -hide-package option the only thing I can do to avoid the ambiguity?
As far as I know, yes. But it doesn't have to be a big deal, you can do this inside ghci:
Prelude Data.List> :set -hide-package mtl
package flags have changed, resetting and loading new packages...
Prelude> import Control.Monad.Cont
Prelude Control.Monad.Cont>
There was also a line-item on GHC-7 change log that made me think package imports would work on the command line, but it doesn't seem to yet (see below). The change comment said something like "full import syntax supported in GHCi", which must exclude extensions I suppose.
$ ghci-7.0.0.20100924 -XPackageImports
GHCi, version 7.0.0.20100924: http://www.haskell.org/ghc/ :? for help
...
Prelude Data.List> import "mtl" Control.Monad.Cont
<no location info>:
Ambiguous module name `Control.Monad.Cont':
it was found in multiple packages: mtl-1.1.1.0 monads-fd-0.1.0.2
Is setting the -hide-package option the only thing I can do to avoid the ambiguity?
You can use ghc-pkg, e.g.
$ ghc-pkg hide monads-fd
This is like setting -hide-package on every subsequent ghc invocation. Packages explicitly depending on monads-fd via Cabal will not be affected, but everything else is. Watch out!
I am trying to teach myself Haskell from the book Learn You A Haskell for Great Good. I got up to the last section of chapter 7 (Modules), where it tells how to create your own module. I did a copy and paste of the Geometry module given in the book at the beginning of the section. The name of the file is Geometry.hs, as the book suggested, and the file is in the bin directory for ghci, which is where I previously was able to successfully do a load using :l for another .hs file.
When I type the following command in GHCi
import Geometry
I get the following error:
Could not find module 'Geometry' It is not a module in the current
program or in any known package
I must be doing something that is obviously wrong, but I can't figure out what it is.
When you use import ModuleName in GHCi, it works (mostly) in the same way import Data.List works: GHC checks your local package database for the module, loads it, and brings its (exported) contents into scope.
However, Geometry isn't a module of a package installed with ghc-pkg. Therefore, GHC doesn't know that a module Geometry exists at all. Neither does it interactive variant GHCi.
But if you :load a program, things change. GHC will take its used modules into account:
-- Foo.hs
module Foo where
foo :: IO ()
foo = putStrLn "Hello from foo!"
-- Main.hs
module Main where
import Foo (foo)
main :: IO ()
main = foo
$ cd /path/to/your/files
$ ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help
Prelude> import Foo
<no location info>:
Could not find module ‘Foo’
It is not a module in the current program, or in any known package.
Prelude> :l Main.hs
[1 of 2] Compiling Foo ( Foo.hs, interpreted )
[2 of 2] Compiling Main ( Main.hs, interpreted )
Ok, modules loaded: Main, Foo.
*Main> :l Main.hs
*Main> foo
Hello from foo!
*Main> import Foo
*Main Foo> -- module now loaded
As you can see, importing Foo first failed. However, after we've actually loaded the program that uses Foo, we were able to use import Foo in GHCi.
So if you want to use import in GHCi, make sure that GHC can find your module, either by including it in a wrapper or installing it. If you just want to load the module itself, use :load.
TLDR: the Learn you a Haskell book fails to mention that you have to :load the Geometry.hs file first. Then :m to go back to Prelude and then import Geometry works.
It is now also possible to add the lib flag when installing packages, i.e. to run cabal install --lib packagename and then to import the corresponding package directly in GHCi. In the present case, for example cabal install --lib hgeometry would facilitate importing modules from this geometry package.