--allow-newer suddenly requires Cabal >=1.19.2 to be available

[edsko]

# cabal install pandoc-1.12.4.2 -j1 --avoid-reinstalls --allow-newer --constraint texmath==0.6.7
Resolving dependencies...
Failed to install pandoc-1.12.4.2
cabal: Error: some packages failed to install:
pandoc-1.12.4.2 failed during the configure step. The exception was:
user error (The package 'pandoc' requires Cabal library version >=1.19.2 &&
>=1.10 but no suitable version is installed.)

[scratchpad-783 ~/env]
# ghc-pkg list | grep Cabal
    Cabal-1.18.1.3

This seems a spurious requirement, and is annoying because it very useful that the cabal binary can be copied from one Haskell install to another without bootstrapping it (more specifically, without copying anything else -- just the binary).

[23Skidoo]

This seems a spurious requirement

No, Setup.hs scripts compiled against Cabal < 1.19.2 don't support --exact-configuration which is needed for --allow-newer to work.

and is annoying because it very useful that the cabal binary can be copied from one Haskell install to another without bootstrapping it (more specifically, without copying anything else -- just the binary).

Note that you can use cabal-install itself to install a newer version of Cabal.

[mietek]

Can we agree the error message is confusing, and should be improved?

I just ran into this while trying to get hscurses to build with GHC 7.8:

$ cabal sandbox init
Writing a default package environment file to
/private/tmp/allow/cabal.sandbox.config
Using an existing sandbox located at /private/tmp/allow/.cabal-sandbox

$ cabal install --allow-newer=unix hscurses
Resolving dependencies...
Notice: installing into a sandbox located at /private/tmp/allow/.cabal-sandbox
Configuring transformers-0.4.1.0...
Failed to install transformers-0.4.1.0
Build log ( /private/tmp/allow/.cabal-sandbox/logs/transformers-0.4.1.0.log ):
cabal: Error: some packages failed to install:
hscurses-1.4.1.2 depends on transformers-0.4.1.0 which failed to install.
mtl-2.2.1 depends on transformers-0.4.1.0 which failed to install.
transformers-0.4.1.0 failed during the configure step. The exception was:
user error (The package 'transformers' requires Cabal library version >=1.19.2
&& >=1.6 but no suitable version is installed.)

This stumped me for a while, as transformers does not actually require the Cabal library. Finally, I realised allow-newer is a recent addition to Cabal, and therefore the error message is most likely spurious.

[23Skidoo]

Reopening: error message should be improved.

[ulysses4ever]

Couple notes:

• The source of the error message:

  cabal/cabal-install/src/Distribution/Client/SetupWrapper.hs

  Lines 683 to 686 in 45b2c34

  	[] -> die' verbosity $ "The package '" ++ prettyShow (packageName pkg)
  	++ "' requires Cabal library version "
  	++ prettyShow (useCabalVersion options)
  	++ " but no suitable version is installed."

• The restriction with 1.19.2 if allow-newer/older specified is implemented here:

  cabal/cabal-install/src/Distribution/Client/Configure.hs

  Lines 99 to 101 in 45b2c34

  	defaultVersionRange = if allowOlder || allowNewer
  	then orLaterVersion (mkVersion [1,19,2])
  	else anyVersion

The error message could be something like: Current build configuration/flags requires Cabal library version >=1.19.2 && >=1.10 to build 'pandoc', but no suitable version is installed (instead of The package 'pandoc' requires Cabal library version >=1.19.2 && >=1.10 but no suitable version is installed). But I'm not sure if this message can appear under circumstances other than those presented in this issue (i.e. --allow-newer+ only Cabal < 1.19.2 available).

Another way to fight it could be to drop the check (referenced in the second bullet-point above) altogether on the grounds that cabal-install-3.x and future would hardly build with Cabal < 1.19.2. How does that sound?

[Mikolaj]

Another way to fight it could be to drop the check (referenced in the second bullet-point above) altogether on the grounds that cabal-install-3.x and future would hardly build with Cabal < 1.19.2. How does that sound?

That's a good idea. Cabal 1.19.2 is way beyond our 5 years support window (https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md). We are currently on the verge of dropping 1.22 or 1.24 IIRC.