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

[Tikhon Jelvis]

Wild guess—could there be an inconsistency between the version of the
binary package the text library was built against and the version you got
the class from in your failing example?

I don't see how that would happen given your setup, but it's also the only
way I can imagine seeing that particular error.

On Tue, Nov 17, 2020, 15:57 amindfv--- via Haskell-Cafe <
haskell-cafe at haskell.org> 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.
>
>     $ cat test.hs
>     import qualified Data.Text as T
>     import qualified Data.Binary as B
>
>     main = print $ B.encode $ T.pack "hello"
>
>     $ /usr/local/bin/ghci --version
>     The Glorious Glasgow Haskell Compilation System, version 8.10.2
>
>     $ /usr/local/bin/ghci 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’
>         • In the second argument of ‘($)’, namely
>             ‘B.encode $ T.pack "hello"’
>           In the expression: print $ B.encode $ T.pack "hello"
>           In an equation for ‘main’: main = print $ B.encode $ T.pack
> "hello"
>       |
>     4 | main = print $ B.encode $ T.pack "hello"
>       |                ^^^^^^^^^^^^^^^^^^^^^^^^^
>     Failed, no modules loaded.
>
>     $ /usr/local/bin/ghc-pkg list text
>     /usr/local/lib/ghc-8.10.2/package.conf.d
>         text-1.2.3.2
>     /home/name/.ghc/x86_64-linux-8.10.2/package.conf.d
>         text-1.2.4.1
>
>     $ ghc-pkg-8.10.2 list text
>     [same result]
>
>     $ /usr/local/bin/ghci -package text-1.2.3.2 test.hs
>     GHCi, version 8.10.2: https://www.haskell.org/ghc/  :? for help
>     [1 of 1] Compiling Main             ( test.hs, interpreted )
>     Ok, one module loaded.
>
>     $ /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’
>         • In the second argument of ‘($)’, namely
>             ‘B.encode $ T.pack "hello"’
>           In the expression: print $ B.encode $ T.pack "hello"
>           In an equation for ‘main’: main = print $ B.encode $ T.pack
> "hello"
>       |
>     4 | main = print $ B.encode $ T.pack "hello"
>       |                ^^^^^^^^^^^^^^^^^^^^^^^^^
>     Failed, no modules loaded.
>
> Both text-1.2.3.2 and text-1.2.4.1 have a Binary instance for Text. What's
> going on?
>
> (After writing this out I rebuilt an unmodified clone of text-1.2.4.1 but
> I see the same results.)
>
> Thanks,
> Tom
>
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