> 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
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