Stack script fails to find pandoc

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#1

This silly script:

#!/usr/bin/env stack
{- stack script --resolver lts-21.25 --package pandoc -}

import Prelude
import Text.Pandoc

main :: IO ()
main = putStrLn "Pandoc detected"

is failing to execute with error:

❯ ./example.hs

/home/cebrian/borralodedentro/example.hs:5:1: error:
    Could not load module ‘Text.Pandoc’
    It is a member of the hidden package ‘pandoc-3.0.1’.
    You can run ‘:set -package pandoc’ 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.
  |
5 | import Text.Pandoc
  | ^^^^^^^^^^^^^^^^^^

and I don’t know why because this is a trimmed down version of another script importing a bunch of libraries with same procedure, only pandoc is failing.

Any ideas?

#2

No idea, haven’t used stack for scripts yet.
Does it do the same if you run stack your-script.hs instead of ./your-script.hs?

#3

Same result here, and you’re right this failure is silly. If I change the script command to runghc it works.

#4

Reproducible. Verbosity uncovers a different error message, “cannot satisfy -package pandoc”.

$ stack --verbose example.hs
...
 [debug] Run process: /.../.ghcup/ghc/9.4.8/bin/runghc-9.4.8 -i -i/.../ -hide-all-packages -fdiagnostics-color=always -packagebase -packagepandoc /.../example.hs

/.../example.hs:5:1: error:
    Could not load module ‘Text.Pandoc’
    It is a member of the hidden package ‘pandoc-3.0.1’.
    You can run ‘:set -package pandoc’ 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.
  |
5 | import Text.Pandoc
  | ^^^^^^^^^^^^^^^^^^

Running that command in isolation shows that something is missing (another dependency?):

$ runghc-9.4.8 -v -hide-all-packages -packagebase -packagepandoc example.hs
Glasgow Haskell Compiler, Version 9.4.8, stage 2 booted by GHC version 9.2.2
<command line>: cannot satisfy -package pandoc
    (use -v for more information)
#5

I’ve filed a bug report Stack script fails to use Pandoc library · Issue #6616 · commercialhaskell/stack · GitHub since this doesn’t seem like normal behaviour.

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#6

Does your script do the same if you change s/pandoc/aeson/ and s/Text.Pandoc/Data.Aeson/?

#7

I could not reproduce this on Windows. With example.hs:

{- stack script --resolver lts-21.25 --package pandoc -}

import Prelude
import Text.Pandoc

main :: IO ()
main = putStrLn "Pandoc detected"

stack example.hs behaves as expected.

(stack --version - Version 2.15.7, Git revision f590e0165b5ab92f5f7f87f8aa55852e1972ee25 x86_64 hpack-0.36.0)

However, I can reproduce it on Ubuntu (via WSL2). In both cases, the final command run by Stack ‘under the hood’ is identical:

Windows:
Run process: D:\sr\programs\x86_64-windows\ghc-9.4.8\bin\runghc-9.4.8.exe -i -i...\ -hide-all-packages -fdiagnostics-color=always -packagebase -packagepandoc ...\example.hs

Linux:
Run process: /home/mpilgrem/.stack/programs/x86_64-linux/ghc-tinfo6-9.4.8/bin/runghc-9.4.8 -i -i.../ -hide-all-packages -fdiagnostics-color=always -packagebase -packagepandoc .../example.hs

So, I am wondering if this is a bug in runghc-9.4.8 on Linux.

#8

If I use GHC 9.4.7 instead (lts-21.21 - which specifies the same version of pandoc), I can’t reproduce the problem on Ubuntu (via WSL2):

{- stack --verbose script --resolver lts-21.21 --package pandoc -}

import Prelude
import Text.Pandoc

main :: IO ()
main = putStrLn "Pandoc detected"

EDT: lts-21.25 has (extract):

- hackage: pandoc-3.0.1@sha256:b86cff1afae695247ee180ac66769881a4900ea4643c87820a9f653dd06d4cf2,37767
  pantry-tree:
    sha256: 4ceae05f232ec6e0a062b36d89da99ad071ef8f498b141ac748c219136b789d3
    size: 141318

lts-21.21 has (extract), which is identical:

- hackage: pandoc-3.0.1@sha256:b86cff1afae695247ee180ac66769881a4900ea4643c87820a9f653dd06d4cf2,37767
  pantry-tree:
    sha256: 4ceae05f232ec6e0a062b36d89da99ad071ef8f498b141ac748c219136b789d3
    size: 141318

EDIT: If I explore with --ghc-options -v, the one that works has:

Loading unit pandoc-3.0.1 ... linking ... done.

and the one that does not work does not. (The one that works also loads lots of other units that the one that does not work does not.)

Both start with (where x is the GHC minor version):

*** initializing unit database:
package flags [-package base{package base True ([])},
               -package pandoc{package pandoc True ([])}]
loading package database /home/mpilgrem/.stack/programs/x86_64-linux/ghc-tinfo6-9.4.x/lib/ghc-9.4.x/lib/package.conf.d
loading package database /home/mpilgrem/.stack/snapshots/x86_64-linux-tinfo6/<hash>/9.4.x/pkgdb
loading package database /home/mpilgrem/.stack/.stack-work/install/x86_64-linux-tinfo6/<hash>/9.4.x/pkgdb

and stack --resolver lts-21.nn exec -- ghc-pkg list pandoc (where nn is the relevant index) shows that that package database does expose pandoc-3.0.1.

EDIT2: It is something about the Linux/GHC 9.4.8/pandoc combination. If I pick, say, ansi-terminal, there is no problem with Linux/GHC 9.4.8. So, this fails on Linux:

{- stack script
   --verbose
   --snapshot lts-21.25
-}

import Text.Pandoc

main :: IO ()
main = putStrLn "Pandoc detected"

but this works on Linux:

{- stack script
   --verbose
   --snapshot lts-21.25
-}

import System.Console.ANSI

main :: IO ()
main = putStrLn "ansi-terminal detected"

as does this (stack is quite a large package, in terms of modules and dependencies):

{- stack script
   --verbose
   --snapshot lts-21.25
-}

import Paths_stack -- From stack package

main :: IO ()
main = print version

EDIT: There does not seem to be a problem with ‘normal’ builds of pandoc; it seems to be something with runghc which (I believe) uses the interactive mode of GHC ‘under the hood’. My tests of ‘normal’ builds have used:

package.yaml:

name: example
version: 1.0.0

dependencies:
- base
- pandoc

executables:
  example:
    main: example.hs

example.hs:

module Main ( main ) where

import Text.Pandoc

main :: IO ()
main = print pandocVersion

stack.yaml

snapshot: lts-21.25 # GHC 9.4.8

and varying the snapshot at the command line.

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#9

So, this is the nub of the paradox on Linux with runghc from GHC 9.4.8:

$ stack --snapshot lts-21.25 exec -- /home/mpilgrem/.stack/programs/x86_64-linux/ghc-tinfo6-9.4.8/bin/ghc-pkg list pandoc --simple-output

pandoc-3.0.1

$ stack --snapshot lts-21.25 exec -- /home/mpilgrem/.stack/programs/x86_64-linux/ghc-tinfo6-9.4.8/bin/runghc-9.4.8 -hide-all-packages -package=base -package=pandoc /home/mpilgrem/code/has
kell/script-test/example1.hs

/home/mpilgrem/code/haskell/script-test/example1.hs:8:1: error:
    Could not load module ‘Text.Pandoc’
    It is a member of the hidden package ‘pandoc-3.0.1’.
    You can run ‘:set -package pandoc’ 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.
  |
8 | import Text.Pandoc
  | ^^^^^^^^^^^^^^^^^^

I have raised: #25035: `runghc` can't find exposed `pandoc` package that `ghc-pkg` lists · Issues · Glasgow Haskell Compiler / GHC · GitLab

EDIT2: The reason I have raised this as a GHC issue is becuase I understand (a) -package=pandoc passed to runghc means ‘pass this same option on to GHC’ and (b) -package=pandoc passed to GHC means ‘cause the identified package to be exposed’. So, it should not be open to runghc to warn that pandoc-3.0.1 is hidden if it has been passed -package=pandoc.

EDIT: I am continuing to experiment with this, and getting some really odd outcomes. I am going to explore if older versions of Stack behave the same way.

EDIT3: I can reduce the nub further, having looked at the source code for runghc. On Ubuntu (via WSL2), with example.hs:

import Text.Pandoc

main :: IO ()
main = print pandocVersion

$ # GHC 9.4.8 ... does not work ...
$ stack --snapshot lts-21.25 exec -- ghc -hide-all-packages -package=base -package=pandoc example.hs

example.hs:1:1: error:
    Could not load module ‘Text.Pandoc’
    It is a member of the hidden package ‘pandoc-3.0.1’.
    You can run ‘:set -package pandoc’ 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 | import Text.Pandoc
  | ^^^^^^^^^^^^^^^^^^

$ # GHC 9.4.7 ... does work ...
$ stack --snapshot lts-21.21 exec -- ghc -hide-all-packages -package=base -package=pandoc example.hs

[1 of 2] Compiling Main             ( example.hs, example.o )
[2 of 2] Linking example

(To confirm, it is something to do with -package=pandoc not exposing pandoc-3.0.1, because:

$ stack --snapshot lts-21.25 exec -- ghc example.hs
$ stack --snapshot lts-21.21 exec -- ghc example.hs

are both fine.)

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