ghci can't load local Prelude module

changed weight to 5

I once reported this in IRC channel but IIRC someone had convinced me that this is not a bug. Maybe that person can write the explanation here. (I don't remember who)

osa1, I am not that person, but here is an explanation. If you have a file called ./Foo.hs and you try to import it, ghci tells you that it doesn't know about that file, because you haven't loaded it yet:

$ 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.

The solution is to run :load Foo.hs before import Foo.

If you have a file called ./Data/List.hs, this now conflicts with the Data.List from base, so you get a different message clarifying which module is not loaded:

$ ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> import Data.List
<interactive>:1:1:
    attempting to use module ‘Data.List’ (./Data/List.hs) which is not loaded

The solution is to run :load Data/List.hs before import Data.List.

Now without -XNoImplicitPrelude, ghci begins by running import Prelude, which normally succeeds by loading the Prelude from base, but in this case, since there is a ./Prelude.hs file, ghci tells you that it can't use that file because it isn't loaded yet.

Since the user didn't explicitly type import Prelude, the error message is confusing, so maybe there should be some special logic to give a different message in that case or even to implicitly :load ./Prelude.hs. Note, however, that there are other modules than Prelude which can trigger this error message when particular files are present and the user hasn't explicitly typed an import statement. If your .ghci has an import Data.List intended to import the Data.List from base, it will usually work, until you have a local file called ./Data/List.hs in which case ghci will complain on startup with this same message that it can't use ./Data/List.hs because it is not loaded.

A workaround which seems to work for hackage-security:

First: write a .ghci file with:

:load src/Prelude.hs
import Prelude
:set -XImplicitPrelude

As cabal doesn't give ability to pass /only/ ghci-flags atm use cabal repl -v or cabal new-repl -v to get a command line cabal uses: something like

/opt/ghc/8.0.1/bin/ghc --interactive -fbuilding-cabal-package -O0 -outputdir ...

then add a -XNoImplicitPrelude there

/opt/ghc/8.0.1/bin/ghc --interactive -XNoImplicitPrelude -fbuilding-cabal-package -O0 -outputdir ...

This works because

if -XNoImplicitPrelude is specified ghci doesn't implicitly load Prelude
after that .ghci file is loaded, where we load right Prelude.hs and import it, and turn implicit prelude on
loading of package proceeds

It seems to work, but fragile it seems.

added Pnormal label

This is certainly a bug.

assigned to @mpickering

Also triggers indirectly from stack repl.

In that case, .ghci is ignored... rather, replaced with stack-generated <XDG_CACHE_HOME>/stack/ghci-script:

:add AppApiserver Definitions.Config Prelude /home/ulidtko/src/foobar/server/app/Apiserver.hs
:module + AppApiserver Definitions.Config Prelude

and there's no viable workaround to get bare stack repl to work; except by -XNoImplicitPrelude, renaming to something like LocalPrelude and explicit imports of it in every module.

Hi @mpickering, can you please assign this one to me?

I've minimized a reproducer against GHC API, and have a fix candidate based on GHC HEAD.

Testing it out currently, and hoping to propose a fix soon.

assigned to @ulidtko

mentioned in merge request !12530 (closed)

I think I fixed it, welcome to review !12530 (closed)

unassigned @mpickering

mentioned in commit e3fdd7ce

mentioned in commit 7c5be613

mentioned in commit 34240c4e

mentioned in commit 3df08208

mentioned in commit c67bcdeb

mentioned in commit 08cb75c4

mentioned in commit 7fd65bf2

mentioned in commit 30e66aa9

closed with commit 977b6b64

mentioned in commit 977b6b64

mentioned in commit 6c1bf036

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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ghci can't load local Prelude module

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