[Haskell-cafe] Why is my 'text' missing an instance?

[Viktor Dukhovni]

On Tue, Nov 17, 2020 at 06:54:58PM -0500, amindfv--- via Haskell-Cafe wrote:

> tl;dr: I seem to have two versions of 'text' on my system, both of
> which should have an instance for (Binary Text). However, one version
> seems to claim not to have that instance.
> 
>     $ /usr/local/bin/ghci -package text-1.2.4.1 test.hs
>     GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
>     [1 of 1] Compiling Main             ( test.hs, interpreted )
>     
>     test.hs:4:16: error:
>         • No instance for (B.Binary T.Text)
>             arising from a use of ‘B.encode’

Hard to say what's different on your system, but on a Fedora 31 system
with GHC 8.10.2 and text-1.2.4.1, I'm unable to reproduce the symptoms
you report.

    $ ghci -package text-1.2.4.1
    GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
    Prelude> import qualified Data.Text as T
    Prelude T> import qualified Data.Binary as B
    Prelude T B> main = print $ B.encode $ T.pack "hello"
    Prelude T B> main
    "\NUL\NUL\NUL\NUL\NUL\NUL\NUL\ENQhello"

The interface file for Text 1.2.4.1 has the expected instances:

    $ ghc-pkg list
    ~/.local/ghc-8.10/lib/ghc-8.10.2/lib/package.conf.d
        ...
        text-1.2.4.1
        ...
    ~/.ghc/x86_64-linux-8.10.2/package.conf.d
        (no packages)

    $ ghc --show-iface ~/.local/ghc-8.10/lib/ghc-8.10.2/lib/x86_64-linux-ghc-8.10.2/text-1.2.4.1/Data/Text.hi | grep '^instance'
    instance [orphan] Data.Binary.Class.Binary [Data.Text.Internal.Text]
    instance [orphan] Data.Data.Data [Data.Text.Internal.Text]
    instance [orphan] GHC.Classes.Eq [Data.Text.Internal.Text]
    instance [orphan] GHC.Exts.IsList [Data.Text.Internal.Text]
    instance [orphan] Data.String.IsString [Data.Text.Internal.Text]
    instance [orphan] Language.Haskell.TH.Syntax.Lift [GHC.Types.LiftedRep,
    instance [orphan] GHC.Base.Monoid [Data.Text.Internal.Text]
    instance [orphan] Control.DeepSeq.NFData [Data.Text.Internal.Text]
    instance [orphan] GHC.Classes.Ord [Data.Text.Internal.Text]
    instance [orphan] Text.Printf.PrintfArg [Data.Text.Internal.Text]
    instance [orphan] GHC.Read.Read [Data.Text.Internal.Text]
    instance [orphan] GHC.Base.Semigroup [Data.Text.Internal.Text]

-- 
    Viktor.