stack ghci ignores flags in stack.yaml and uses package.cabal defaults

[liskin]

Steps to reproduce

Minimal example:

#!/usr/bin/env bash

set -ex

mkdir /tmp/stack-ghci-flags-bug
cd /tmp/stack-ghci-flags-bug

wget https://github.com/commercialhaskell/stack/releases/download/v2.5.1/stack-2.5.1-linux-x86_64.tar.gz
tar xaf stack-2.5.1-linux-x86_64.tar.gz

mkdir -p bug/src

cat >bug/bug.cabal <<END
name:               bug
version:            0.1
cabal-version:      >= 1.10
build-type:         Simple

flag someflag
    default: False

library
    default-language: Haskell2010
    hs-source-dirs:  src

    build-depends: base
    exposed-modules: Bug1

    if flag(someflag)
        build-depends: containers
        other-modules: Bug2
        cpp-options: -DBUG
END

cat >bug/src/Bug1.hs <<END
{-# LANGUAGE CPP #-}
module Bug1 (Maybe) where
import Data.Maybe
#ifdef BUG
import Bug2
#endif
END

cat >bug/src/Bug2.hs <<END
module Bug2 (Map) where import Data.Map
END

cat >stack.yaml <<END
resolver: lts-16.18
packages:
    - bug
flags:
    bug:
        someflag: true
END

./stack-2.5.1-linux-x86_64/stack --stack-root "$PWD"/.stack --system-ghc build
./stack-2.5.1-linux-x86_64/stack --stack-root "$PWD"/.stack --system-ghc ghci bug:lib

This results in

Configuring GHCi with the following packages: bug
GHCi, version 8.8.4: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/tomi/.ghc/ghci.conf
[1 of 2] Compiling Bug1             ( /tmp/stack-ghci-flags-bug/bug/src/Bug1.hs, interpreted )
[2 of 2] Compiling Bug2             ( /tmp/stack-ghci-flags-bug/bug/src/Bug2.hs, interpreted )

/tmp/stack-ghci-flags-bug/bug/src/Bug2.hs:1:25: error:
    Could not load module ‘Data.Map’
    It is a member of the hidden package ‘containers-0.6.2.1’.
    You can run ‘:set -package containers’ to expose it.
    (Note: this unloads all the modules in the current scope.)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
1 | module Bug2 (Map) where import Data.Map
  |                         ^^^^^^^^^^^^^^^
Failed, one module loaded.

<no location info>: error:
    Could not load module ‘Bug2’
    it is a hidden module in the package ‘bug-0.1’
Loaded GHCi configuration from /tmp/haskell-stack-ghci/3889aa92/ghci-script

With ./stack-2.5.1-linux-x86_64/stack --stack-root "$PWD"/.stack --system-ghc ghci --flag bug:someflag bug:lib it works just fine.

Adding -v reveals that -DBUG isn't passed to ghci either. Do note that my expected behaviour is not that Bug2 wouldn't be loaded. My expected behaviour is that the ghci is invoked with -DBUG and -package-id=containers-…, so that I can use ghci and ghcid with the flags that I set in stack.yaml.

Now here's a non-minimal real life example, just to show that this is actually reproducible with a real project, that perhaps some of you folks use and love:

#!/usr/bin/env bash

set -ex

mkdir /tmp/stack-ghci-flags-bug-xmobar
cd /tmp/stack-ghci-flags-bug-xmobar

wget https://github.com/commercialhaskell/stack/releases/download/v2.5.1/stack-2.5.1-linux-x86_64.tar.gz
tar xaf stack-2.5.1-linux-x86_64.tar.gz

git clone https://github.com/jaor/xmobar

cat >stack.yaml <<END
resolver: lts-16.18
packages:
    - xmobar
flags:
    xmobar:
        with_xft: true
END

./stack-2.5.1-linux-x86_64/stack --stack-root "$PWD"/.stack --system-ghc build
./stack-2.5.1-linux-x86_64/stack --stack-root "$PWD"/.stack --system-ghc ghci xmobar:lib

This fails with:

[58 of 67] Compiling Xmobar.X11.MinXft ( /tmp/stack1/xmobar/.stack-work/dist/x86_64-linux/Cabal-3.0.1.0/build/Xmobar/X11/MinXft.hs, interpreted )

src/Xmobar/X11/MinXft.hsc:49:1: error:
    Could not load module ‘Graphics.X11.Xrender’
    It is a member of the hidden package ‘X11-xft-0.3.1’.
    You can run ‘:set -package X11-xft’ to expose it.
    (Note: this unloads all the modules in the current scope.)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
Failed, 57 modules loaded.

<no location info>: error:
    Could not load module ‘Xmobar’
    It is a member of the hidden package ‘xmobar-0.37’.
    You can run ‘:set -package xmobar’ to expose it.
    (Note: this unloads all the modules in the current scope.)

Stack version

$ ./stack-2.5.1-linux-x86_64/stack --version
Version 2.5.1, Git revision d6ab861544918185236cf826cb2028abb266d6d5 x86_64 hpack-0.33.0

It's also reproducible with stack 2.3.3 from Debian.

Method of installation

wget https://github.com/commercialhaskell/stack/releases/download/v2.5.1/stack-2.5.1-linux-x86_64.tar.gz
tar xaf stack-2.5.1-linux-x86_64.tar.gz

The rest of the system is Debian testing on amd64 with ghc 8.8.4.

[jneira]

@liskin many thanks for open a different issue, with so many details.

[mattaudesse]

Thanks for the excellent write-up @liskin 👍