Type problem

[Magicloud Magiclouds]

I have a function as:

hsNames ::

      forall (m :: * -> *). (PersistQuery m,

                             PersistMonadBackend m ~ SqlBackend) =>

      [(String,

        [String],

        Text -> Text -> UTCTime -> UTCTime -> m [[Value]])]

Then I was using it in a handler:

let (_, fs, _) = head hsNames

I got:

    Couldn't match expected type `persistent-1.2.1:Database.Persist.Sql.Types.SqlBackend'

                with actual type `PersistMonadBackend m0'

    The type variable `m0' is ambiguous

    Possible fix: add a type signature that fixes these type variable(s)

    In the second argument of `head', namely `hsNames'

-- 

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[Michael Snoyman]

It's saying that you're not using the third element in the tuple, and therefore it doesn't know what the type variable `m` means. I've written up a short example on SoH to better demonstrate:

https://www.fpcomplete.com/user/snoyberg/random-code-snippets/free-type-variable

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[Magicloud Magiclouds]

I tried to give it a type signature, but failed. Last time I met this issue with exactly the same types, was because I missed calling runDB. But here, how to resolve?

[Michael Snoyman]

I don't know, you haven't told us what type signature you tried to provide.

[Magicloud Magiclouds]

Ah, sorry, it was the type signature for the third one.

let (_, fs, (_ :: What should be here)) = head hsNames

[Michael Snoyman]

OK... what type signature did you try to put in there?

[Magicloud Magiclouds]

I have no idea, because it was not used, so anything compilable is fine.