[Haskell-beginners] type vs data, again...

Andres Löh andres at well-typed.com
Sun Mar 17 14:51:31 CET 2013

Hi again.

>  thank you for the tip! It helps but it's not quite there yet. If you see
> the program in attachment, I can make it compile only by commenting the type
> declaration for the second function, otherwise it's the exact same error
> message as before.

You're probably using the wrong HashMap. There are several packages on
Hackage implementing similarly-named data structures (which is
confusing, I know, but such is life in a decentralized and open
world). You should use what aeson is using (because that's the library
defining the Object type synonym).

Looking at the import list of


we see

import Data.HashMap.Strict (HashMap)

and by checking the package dependencies of aeson, we figure out that
this module is provided by the unordered-containers package.


Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com

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