[Haskell-cafe] Warnings with OverloadedLists

Ozgur Akgun ozgurakgun at gmail.com
Tue Jun 17 21:46:04 UTC 2014

Hi Nikolay,

On 16 June 2014 18:19, Nikolay Amiantov <nikoamia at gmail.com> wrote:

> If I disable OverloadedLists extension, warning goes away. Can someone
> clear this behaviour out for me? It looks like a bug, but I'm not
> sure.

I am not sure if this is a bug or not, but this page seems to explain the
desugaring of patterns with OverloadedLists:

As a workaround, you could try calling `toList` explicitly in a case
statement to implement a `len` function for types with an IsList instance.
Maybe something like the following.

len :: IsList l => l -> Int
len xs = case toList xs of [] -> ...

Or even:

len :: IsList l => l -> Int
len = length . toList

Hope this helps.

Ozgur Akgun
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