ghc error: hidden package, but it's actually exposed - haskell

Question

I'm trying to use the package Parsec.
When I run ghc Main.hs I get the error message:
Could not load module ‘Text.Parsec’
It is a member of the hidden package ‘parsec-3.1.14.0’.
You can run ‘:set -package parsec’ to expose it.
However, the package is supposed to be exposed already: ghc-pkg list gives
usr/lib/ghc/package.conf.d
Agda-2.6.0.1
Cabal-2.4.0.1
...
parsec-3.1.13.0
...
What should I do get ghc load it automatically?
The full error message mentioned both versions of the package:
ExpScanner.hs:13:1: error:
Could not load module ‘Text.Parsec’
It is a member of the hidden package ‘parsec-3.1.14.0’.
You can run ‘:set -package parsec’ to expose it.
(Note: this unloads all the modules in the current scope.)
It is a member of the hidden package ‘parsec-3.1.13.0’.
You can run ‘:set -package parsec’ to expose it.
(Note: this unloads all the modules in the current scope.)
Use -v to see a list of the files searched for.
ExpScanner.hs is a moduled called by Main.
The full output of ghc-pkg list is
/usr/lib/ghc/package.conf.d
Agda-2.6.0.1
Cabal-2.4.0.1
EdisonAPI-1.3.1
EdisonCore-1.3.2.1
GLURaw-2.0.0.4
GLUT-2.7.0.15
HTTP-4000.3.14
HUnit-1.6.0.0
ObjectName-1.1.0.1
OpenGL-3.0.3.0
OpenGLRaw-3.3.3.0
QuickCheck-2.12.6.1
STMonadTrans-0.4.3
StateVar-1.1.1.1
aeson-1.4.2.0
array-0.5.3.0
async-2.2.2
attoparsec-0.13.2.2
base-4.12.0.0
base-compat-0.10.5
binary-0.8.6.0
blaze-builder-0.4.1.0
blaze-html-0.9.1.1
blaze-markup-0.8.2.2
boxes-0.1.5
bytestring-0.10.8.2
call-stack-0.1.0
case-insensitive-1.2.0.11
containers-0.6.0.1
data-hash-0.2.0.1
deepseq-1.4.4.0
directory-1.3.3.0
dlist-0.8.0.6
edit-distance-0.2.2.1
equivalence-0.3.4
erf-2.0.0.0
exceptions-0.10.2
fgl-5.7.0.1
filepath-1.4.2.1
fixed-0.2.1.1
geniplate-mirror-0.7.6
ghc-8.6.5
ghc-boot-8.6.5
ghc-boot-th-8.6.5
ghc-compact-0.1.0.0
ghc-heap-8.6.5
ghc-prim-0.5.3
ghci-8.6.5
gitrev-1.3.1
half-0.3
hashable-1.2.7.0
hashtables-1.2.3.3
haskeline-0.7.4.3
haskell-src-1.0.3.0
hpc-0.6.0.3
html-1.0.1.2
ieee754-0.8.0
integer-gmp-1.0.2.0
integer-logarithms-1.0.3
libiserv-8.6.3
mtl-2.2.2
murmur-hash-0.1.0.9
network-2.8.0.1
network-uri-2.6.1.0
parallel-3.2.2.0
parsec-3.1.13.0
pretty-1.1.3.6
primitive-0.6.4.0
process-1.6.5.0
random-1.1
regex-base-0.93.2
regex-compat-0.95.1
regex-posix-0.95.2
regex-tdfa-1.2.3.2
rts-1.0
scientific-0.3.6.2
split-0.2.3.3
stm-2.5.0.0
strict-0.3.2
syb-0.7.1
tagged-0.8.6
template-haskell-2.14.0.0
terminfo-0.4.1.2
text-1.2.3.1
tf-random-0.5
th-abstraction-0.2.11.0
time-1.8.0.2
time-locale-compat-0.1.1.5
transformers-0.5.6.2
transformers-compat-0.6.5
unix-2.7.2.2
unordered-containers-0.2.10.0
uri-encode-1.5.0.5
utf8-string-1.0.1.1
uuid-types-1.0.3
vector-0.12.0.3
xhtml-3000.2.2.1
zlib-0.6.2
/home/venanzio/.ghc/x86_64-linux-8.6.5/package.conf.d
Cabal-3.2.0.0
base16-bytestring-0.1.1.6
base64-bytestring-1.0.0.3
conway-0.1.0.0
cryptohash-sha256-0.11.101.0
echo-0.1.3
ed25519-0.0.5.0
hackage-security-0.6.0.1
lukko-0.1.1.2
resolv-0.1.2.0
tar-0.5.1.1

Answer 1

This looks like an issue with global vs local installs. Oh, and there it is in your ghc-pkg list output. You've got a multiuser ghc install and a single-user list of packages you've installed. Things work when you run ghc as a superuser because they won't see your local (per-user) installs.
This is going to cause problems unless you use a tool to manage your environment for you. Both cabal and stack can handle this fine. I prefer cabal because it doesn't need coaxing to work with your preinstalled GHC, but this is a matter that has caused religious wars in the past. I won't argue against stack if you have a good resource for using it instead.