Generic Parameter 'T'could not be inferred about grdb.swift HOT 7 CLOSED

Hello @mtissington

There are two ways to use the inDatabase method:

// Without any return value
dbQueue.inDatabase { db in
    // do some stuff
}

// With a return value
let value = dbQueue.inDatabase { db in
    return ... // return some stuff
}

In your case, you return a value, but don't use it outside: the poor Swift compiler infers that the inDatabase call should both have a value (returned from inside), and have no value (since nothing is consumed), and collapses. I tend to think it's a Swift bug.

The fix is not to return anything, or to use the returned value.

Since you're the second person who asks that question, I think I should at least update the FAQ ;-)

from grdb.swift.

@mtissington, I've just been bitten in an unexpected way myself. Sometimes you also have to explicitly declare the type of the returned value:

// Variant 1
let value: MyType = dbQueue.inDatabase { db in
    return ...
}

// Variant 2
let value = dbQueue.inDatabase { db -> MyType in
    return ...
}

I have to do something about this: I reopen this issue.

from grdb.swift.

as an fyi i like how you have packaged this all up, for the most part it works quick and is easy to us. thanks!

from grdb.swift.

@mtissington That's the best thing a library writer can read 😄 Happy GRDB!

from grdb.swift.

Little update: the problem is still there in Swift 3, and I could not find any workaround. At least I have enough matter for a bug report on http://bugs.swift.org.

from grdb.swift.

Swift issue created: https://bugs.swift.org/browse/SR-1570

from grdb.swift.

@mtissington I close this issue: there's nothing I can do but waiting for SR-1570 to be fixed. I hope the FAQ will help other users. Thanks for the report!

from grdb.swift.