[Haskell-cafe] Non-exhaustive pattern match warning (incorrect?)

[Antoine Latter]

On Mon, Dec 26, 2011 at 2:19 PM, Michael Orlitzky <michael at orlitzky.com> wrote:
> On 12/26/11 13:42, Antoine Latter wrote:
>>>
>>> Am I overlooking something, or did I already match Octet.None?
>>>
>>
>> What is your definition of the 'Octet' type?
>>
>
> -- An Octet consists of eight bits. For our purposes, the most
> -- significant bit will come "first." That is, b1 is in the 2^7
> -- place while b8 is in the 2^0 place.
> data Octet = None | Octet { b1 :: Bit,
>                            b2 :: Bit,
>                            b3 :: Bit,
>                            b4 :: Bit,
>                            b5 :: Bit,
>                            b6 :: Bit,
>                            b7 :: Bit,
>                            b8 :: Bit }
>           deriving (Eq)
>

The error is warning you that the record update 'oct { b8 = bit }' can
fail at run-time if 'oct' is None.

Since it looks like you've checked for that you shouldn't have a
problem, but the compiler doesn't know that.

If you decompose your type into 'Octet' without the 'None' case, and
'Maybe Octet' for the times when 'None' is appropriate, the compiler
will have enough information to not give warnings like this.

I can't be the one to tell you if that is worth it or not.

Antoine

>
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