Failed to build Agda 2.6.1.2 using GHC 8.10.5

[Nils Anders Danielsson]

I tried to build Agda 2.6.1.2 using GHC 8.10.5 and encountered the following error:

src/full/Agda/Utils/Update.hs:23:1: error:
    Could not load module ‘Control.Monad.Trans.Identity’
    It is a member of the hidden package ‘transformers-0.5.6.2’.
    Perhaps you need to add ‘transformers’ to the build-depends in your .cabal file.
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.

I used the following command:

cabal install Agda-2.6.1.2 --program-suffix=-2.6.1.2 -fenable-cluster-counting -foptimise-heavily --with-compiler=<ghc-8.10.5>

The problem seems to come from the following part of Agda.cabal:

agda/Agda.cabal

Lines 242 to 246 in 022d31c

 -- In hTags the mtl library must be compiled with the version of 
 -- transformers shipped with GHC, so we use that version in Agda (see, 
 -- for example, Issue #2983). 
 if impl(ghc >= 8.6.4) && impl(ghc < 8.10.3) 
   build-depends: transformers == 0.5.6.2 

I think we should try to ensure that if Cabal manages to satisfy all constraints, then Agda builds. In this case Cabal started building Agda, but failed when building module 147. If constraints like impl(ghc < 8.10.3) are included, then I think we should also ensure that Cabal does not even try to build Agda for GHC 8.10.$n$, where $n ≥ 3$.

[Andreas Abel]

I tried to build Agda 2.6.1.2 using GHC 8.10.5

Agda-2.6.1.2 is explicitly deprecated. So we do not fix it.
In 2.6.1.3 the problem seems fixed.

if impl(ghc>8.10.3)
transformers (>=0.5.6.2)
if impl(ghc>=8.6.4) && impl(ghc<=8.10.3)
transformers (==0.5.6.2)
if impl(ghc>=8.4) && impl(ghc<8.6.4)
transformers (==0.5.5.0)
if impl(ghc<8.4)
transformers (==0.5.2.0)

I think we should try to ensure that if Cabal manages to satisfy all constraints, then Agda builds.

I agree.

Current Agda.cabal does not have the problem of minor GHC version bounds that get outdated by new GHC releases anymore. I dimly remember there was a discussion with @asr that this would be brittle.

[Andreas Abel]

I think you rediscovered this issue:

• #5136

[Nils Anders Danielsson]

I searched for things like impl ghc, 'impl(ghc' and transformers 0.5.6.2, but that issue was not part of the results. Could it be that if one links to code in an issue, in such a way that the code is displayed inline, then that code is ignored by the search feature? Is there a way to get the search feature to include the code?

[Andreas Abel]

I am using search only naively, and I am usually puzzled by the search results.