The manual should be the best place.
However, the questions you ask, i.e.
- "what is a package database"
- "how does writing a minimal package definition make a sandbox that
protects you from X and Y"
are wrong questions. As user of cabal-install you should not worry about
package databases (which are really a GHC concept) nor really think of
sandboxes (as there aren't any in the direct meaning).
Instead, in my opinion, the right (and only) question you should ask is
"how to write .cabal package definition"
which is answered in
https://cabal.readthedocs.io/en/stable/how-to-package-haskell-code.html
I personally don't like `cabal init` wizard, but it's ok if you are not
too familiar with .cabal files.
- Oleg
On 4/15/26 13:26, Simon Peyton Jones wrote:
Thanks Oleg.
Where is the best place to read about cabal's mental model of package
databases (global and local), project files. etc? Just basic things
like "what is a package database" and "how does writing a minimal
package definition make a sandbox that protects you from X and Y".
I had a look at https://cabal.readthedocs.io/en/stable/cabal-context.html
which is a good start, but doesn't explain enough concepts.
Is there anything else I should look at?
Thanks
Simon
On Tue, 14 Apr 2026 at 17:31, Oleg Grenrus via ghc-devs
<[email protected]> wrote:
> why ghc-22 can know about `base`
Because these are part of global package database. And since a
decade ago cabal-install treats global package database as
immutable. And very likely if you modify package database in
_build, hadrian might get very confused.
Yes, the immutability of global package database might be
inconvenient for the very few GHC hackers, but by making it
practically impossible to modify that database saves a lot of
people from (accidentally) messing their setups.
>> Why you want to build and install a bunch of libraries? You
most likely don't want to do that.
> I think I really do
I argue that if you don't really feel that you need to amend
global package database of your "main" global/default GHC, then
you don't need to amend the global package database of your
WIP-GHC either. It might be the very case that you don't work on
"ordinary" projects i.e. which use cabal as their build tool but
are intended to be compiled with different GHC versions, so you
don't feel at home using cabal-install, writing package & project
definitions and changing to different GHC versions at will. But
please trust me that the non-stateless workflow of cabal-install
is really a lot better long term and actually allows you do more stuff
Allow `cabal-install` manage (the local) package database for you.
> All those see a bit complicated wrt
FWIW, I also think that the invocations Tom mentioned are a lot
more complicated compared to writing minimal package definition
cabal-version: 3.0
name: my-test
version: 0
library
build-depends: base, hspec
exposed-modules: Foo.hs
and `cabal build -w $HOME/code/HEAD-22/_build/.../ghc` and `cabal
build -w ghc` to compare the behavior with stock GHC etc.
There are those six or so lines of "boilerplate setup", but they
actually do declare dependencies, so in case you have to return to
this example later, or share it with someone, it's all there in
"runnable" format.
- Oleg
On 4/10/26 22:09, Simon Peyton Jones via ghc-devs wrote:
Thank you. All those see a bit complicated wrt
ghc-22 Foo.hs -package hspec
I am curious about why ghc-22 can know about `base` and
`containter` (all safely tucked away in
$HOME/code/HEAD-22/_build) but not about `hspec`.
Simon
On Fri, 10 Apr 2026 at 14:47, Tom Smeding via ghc-devs
<[email protected]> wrote:
Dear Simon,
I do not know the answer to your question, but I do know a
few workarounds using cabal that may or may not be helpful.
Cabal lets you do this:
$ cabal repl -b hspec -w ~/code/HEAD-22/_build/stage1/bin/ghc
this creates a fake project with "hspec" as the list of
dependencies, and starts a ghci session there. (You could
also write something like `cabal repl -b 'hspec ==2.11.17,
HUnit >= 1.6'` -- it's really a list of constraints.)
Naturally, this doesn't work if you want to compile a file
instead of opening ghci.
One can work around this as follows (but see the warning below):
$ cabal repl -b hspec -w ~/code/HEAD-22/_build/stage1/bin/ghc
--repl-option=-fobject-code --repl-option=Foo.hs
which does open ghci, but it compiles Foo.hs to an object
file, which may sufficient for your use case. Warning: it
seems that -fobject-code breaks the illusion that the fake
project doesn't actually exist, as a `dist-newstyle` tree is
created in the current directory, which you may need to clean
up afterwards.
Note: I used --repl-option twice instead of --repl-options
(mind the S) once to allow spaces in Foo.hs.
Another workaround which does allow you to avoid opening ghci
but requires editing the test file, is to make it a "cabal
script". [1] If I put the below (between the markers) in a
file `test.hs`:
```
{- cabal:
build-depends: base, hspec
-}
{- project:
with-compiler: /path/to/some/ghc
-}
module M where
x = 4
```
and run:
$ cabal build test.hs
the thing is compiled to an object file, and I get an error
from GHC that there was no Main module but there was a -o
option so what did you want to do exactly. (If test.hs is a
Main module, this works better, naturally).
Apologies if this is not helpful, but perhaps at least one of
these tricks was new to one of the readers of this list.
- Tom
[1]:
https://cabal.readthedocs.io/en/stable/cabal-commands.html#cabal-run
On 10/04/2026 09:22, Simon Peyton Jones via ghc-devs wrote:
Why you want to build and install a bunch of libraries?
You most likely don't want to do that.
I think I really do. Let's call my build #22 ghc-22
export ghc-22 $HOME/code/HEAD-22/_build/stage1/bin/ghc
Then on any one of dozens of 5-line tests, given in tickets,
I can say
ghc-22 -c Foo.hs
and ghc-22 already knows about base, ghc-internal, text,
containers etc built by and for ghc-22. They are squirrelled
away somewhere in $HOME/code/HEAD-22/_build
It's like "batteries included": I already have `base`
Now some has a test that needs `hspec`. I'd like to add
`hspec` to the batteries in $HOME/code/HEAD-22/_build, so
that after that I can always say
ghc-22 -package hspec Foo.hs
and away we go.
Yes I could make a cabal project for a 5-line test, but
that's more keystrokes. Is it difficult to just get it to
treat `hspec` the same way that it treats `base` or
`containers`?
I know this isn't the intended use-case for cabal; it's just
the use-case I have.
Thanks
Simon
On Fri, 10 Apr 2026 at 05:01, Oleg Grenrus via ghc-devs
<[email protected]> wrote:
Why you want to build and install a bunch of libraries?
You most likely don't want to do that.
If you want to play with particular GHC version, create
an ordinary cabal package with dependencies you need,
and point `cabal-install` to use your
HOME/code/HEAD-22/_build/stage1/bin/ghc
There is nothing (noteworthy) special about `cabal repl
-w $HOME/code/HEAD-22/_build/stage1/bin/ghc`; as long as
`cabal-install` is concerned, it's just some GHC build.
- Oleg
On 4/8/26 17:43, Simon Peyton Jones via ghc-devs wrote:
Dear devs
I have thirty or so builds of GHC on my disk.
Sometimes I want to use one build to build and install
(for that build alone) a bunch of libraries. If I do
cabal install hspec -w
$HOME/code/HEAD-22/_build/stage1/bin/ghc
then Cabal rightly warns me
Warning: The libraries were installed by creating a
global GHC environment
file at:
/home/simonpj/.ghc/x86_64-linux-9.15.20260309/environments/default
The presence of such an environment file is likely
to confuse or break other
tools because it changes GHC's behaviour: it
changes the default package set
in ghc and ghci from its normal value (which is
"all boot libraries"). GHC
environment files are little-used and often not
tested for.
Question: how can I install the libraries in the build
tree for $HOME/code/HEAD-22?
After all, I think ghc-internal, base etc are all in
that build-tree. Surely hspec can be too?
But how?
Thanks!
Simon
_______________________________________________
ghc-devs mailing list [email protected]
To unsubscribe send an email [email protected]
_______________________________________________
ghc-devs mailing list -- [email protected]
To unsubscribe send an email to [email protected]
_______________________________________________
ghc-devs mailing list [email protected]
To unsubscribe send an email [email protected]
_______________________________________________
ghc-devs mailing list -- [email protected]
To unsubscribe send an email to [email protected]
_______________________________________________
ghc-devs mailing list [email protected]
To unsubscribe send an email [email protected]
_______________________________________________
ghc-devs mailing list -- [email protected]
To unsubscribe send an email to [email protected]
_______________________________________________
ghc-devs mailing list -- [email protected]
To unsubscribe send an email to [email protected]
