How do I use PackageImports properly in ghci?

user_668074 0
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I want to do testing in ghci and I'm running into problems with similarly named modules in different packages (this is basically a continuation of my previous question: What should I do if two modules share the same name?)

The packages in question are crypto-api, cryptonite, and DRBG. They all provides modules involving Crypto.Random. I'm using PackageImports in ghci to work fix this issue.

However, it seems that the order of using PackageImports is important.

This works (in ghci):

:set -XPackageImports
import Crypto.Random.DRBG
import "crypto-api" Crypto.Random

This does not work:

:set -XPackageImports
import "crypto-api" Crypto.Random
import Crypto.Random.DRBG  -- Error!

Also this does not work:

:set -XPackageImports
import "crypto-api" Crypto.Random
import "DRBG" Crypto.Random.DRBG -- Error!

The error that appears is:

<interactive>:1:1: error:
Ambiguous interface for ‘Crypto.Random’:
  it was found in multiple packages:
  crypto-api-0.13.2 cryptonite-0.23

Am I doing something wrong or is this a bug?

ghci haskell
2 Answers

Perhaps the easiest way is to avoid PackageImports and instead rename the packages when starting ghci, using the -package option:

ghci -package "crypto-api (Crypto.Random as A)" -package "cryptonite (Crypto.Random as B)"

Once in ghci, you could import the renamed modules:

Prelude> import A
Prelude A> import B
Prelude A B>

The syntax -package "crypto-api (Crypto.Random as A)" makes only the Crypto.Random module available to ghci, but not the other modules in the package.

According to the Thinning and renaming modules section of the GHC user guide:

We also support renaming of modules, in case you need to refer to both modules simultaneously; this is supporting by writing OldModName as NewModName, e.g. -package "base (Data.Bool as Bool). You can also write -package "base with (Data.Bool as Bool) to include all of the original bindings (e.g. the renaming is strictly additive).

So perhaps it's better to write the options like -package "crypto-api with (Crypto.Random as A)" to maintain access to all the modules.

Using PackageImports instead of thinning and renaming modules has the problem that it makes the source code dependent on the precise packages in which imported modules live. If a module changes packages that breaks the program.

user_13642880

Why would you expect that to work at all?

:set -XPackageImports
import "crypto-api" Crypto.Random
import Crypto.Random.DRBG  -- Error!
import "DRBG" Crypto.Random.DRBG -- Error!

You have imported the DRBG module twice and from the same package. From your question I thought you'd want to get the Crypto.Random modules from crypto-api and cryptonite but I don't see any attempt to use cryptonite at all here.

Just so we're clear, I can reproduce the error I think you were getting via GHCi:

Prelude> :set -XPackageImports
Prelude> import "crypto-api" Crypto.Random as OriginalRandom
Prelude OriginalRandom> import "cryptonite" Crypto.Random as ConflictRandom

<interactive>:1:1: error:
    Ambiguous module name ‘Crypto.Random’:
      it was found in multiple packages:
      crypto-api-0.13.2 cryptonite-0.24

But it all works when you load it from a file so this is just a bug and can be avoided:

% cat t.hs
{-# LANGUAGE PackageImports #-}

import "crypto-api" Crypto.Random as OriginalRandom
import "cryptonite" Crypto.Random as ConflictRandom
import Crypto.Random.DRBG -- This only appears in one package
% ghci t.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/tommd/.ghci
[1 of 1] Compiling Main             ( t.hs, interpreted )
Ok, 1 module loaded.
*Main>

Notice that DRBG is not part of this discussion. It doesn't have any conflicting modules names with anything on hackage as far as I know.

user_2161640
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