GHC在cabal沙箱中找不到模块

Question

I am using Haskell version 7.8.4 on (X)Ubuntu 15.10, with Cabal-Install 1.18 both installed through apt . I have not attempted to install anything Haskell-related by hand on this machine. I set up a cabal sandbox, fetched and installed a module only to discover that ghc doesn't seem to be picking it up. ghc -v seems to suggest that I have two versions of the cabal library that are mutually shadowing each other. How does this work?

I am starting out with an empty directory /tmp/haskell-example

then I do a cabal sandbox init .

$ cabal sandbox init
Writing a default package environment file to
/tmp/haskell-example/cabal.sandbox.config
Creating a new sandbox at /tmp/haskell-example/.cabal-sandbox

then I install natural-numbers because I want to use the Data.Natural module in a program. This operation succeeds.

$ cabal install natural-numbers
Resolving dependencies...
Notice: installing into a sandbox located at
/tmp/haskell-example/.cabal-sandbox
Configuring natural-numbers-0.1.2.0...
Building natural-numbers-0.1.2.0...
Installed natural-numbers-0.1.2.0

I can verify that the Data.Natural module has indeed been installed to the cabal sandbox.

$ ls /tmp/haskell-example/.cabal-sandbox/lib/x86_64-linux-ghc-7.8.4/natural-numbers-0.1.2.0
 Data
libHSnatural-numbers-0.1.2.0.a
libHSnatural-numbers-0.1.2.0-ghc7.8.4.so
$ ls /tmp/haskell-example/.cabal-sandbox/lib/x86_64-linux-ghc-7.8.4/natural-numbers-0.1.2.0/Data
Natural.dyn_hi
Natural.hi

then I create a simple Main.hs file that imports Data.Natural but does not use it.

module Main where

import Data.Natural

main = putStrLn "Hello World"

When I try to ghc Main.hs I see the following :

$ ghc Main.hs 
Main.hs:3:8:
    Could not find module ‘Data.Natural’
    Use -v to see a list of the files searched for.

With the verbose flag enabled, it seems as if my cabal is getting shadowed by a later cabal, which is in turn shadowing the earlier cabal. Why is this happening?

$ ghc -v Main.hs 
Glasgow Haskell Compiler, Version 7.8.4, stage 2 booted by GHC version 7.8.4
Using binary package database: /usr/lib/ghc/package.conf.d/package.cache
hiding package Cabal-1.18.1.5 to avoid conflict with later version Cabal-1.22.1.1
wired-in package ghc-prim mapped to ghc-prim-0.3.1.0-ec14d2f6075975a2ce9112eae431c8e1
wired-in package integer-gmp mapped to integer-gmp-0.5.1.0-de4898ebdc5ab81cedce89121ae9ac84
wired-in package base mapped to base-4.7.0.2-5ef1e7e809bc3b18d74efc783356e209
wired-in package rts mapped to builtin_rts
wired-in package template-haskell mapped to template-haskell-2.9.0.0-c1976a420ad8b9b589eee08844c59ba2
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags: 
hiding package Cabal-1.18.1.5 to avoid conflict with later version Cabal-1.22.1.1
wired-in package ghc-prim mapped to ghc-prim-0.3.1.0-ec14d2f6075975a2ce9112eae431c8e1
wired-in package integer-gmp mapped to integer-gmp-0.5.1.0-de4898ebdc5ab81cedce89121ae9ac84
wired-in package base mapped to base-4.7.0.2-5ef1e7e809bc3b18d74efc783356e209
wired-in package rts mapped to builtin_rts
wired-in package template-haskell mapped to template-haskell-2.9.0.0-c1976a420ad8b9b589eee08844c59ba2
wired-in package dph-seq not found.
wired-in package dph-par not found.
*** Chasing dependencies:
Chasing modules from: *Main.hs

Main.hs:3:8:
    Could not find module ‘Data.Natural’
    Locations searched:
    Data/Natural.hs
    Data/Natural.lhs
*** Deleting temp files:
Deleting: 
*** Deleting temp dirs:
Deleting: 

Answer 1

If you're into manual hacking, you can pass to ghc the sandbox's pkg db location, such as:

ghc Main.hs -package-db .cabal-sandbox/x86_64-linux-ghc-7.10.2-packages.conf.d/

However the "normal" way to use sandboxes is to always compile using cabal build (or cabal install with no parameters) rather than running ghc directly.

1. Run cabal init , answer questions as you please
2. Edit the resulting foo.cabal file ( foo is the name of your package).
3. Run cabal build or cabal install - this will run ghc for you.

When editing the cabal file, check that your exported modules (if it's a lib) are listed and that your main src is correct. Also make sure dependencies such as natural-numbers are listed in the build-depends: clause.