[Haskell-cafe] 1 = 0?

[Vo Minh Thu]

2014-06-17 12:51 GMT+02:00 Roman Cheplyaka <roma at ro-che.info>:
> * Alexey Muranov <alexey.muranov at gmail.com> [2014-06-17 03:25:10-0700]
>> Is this the expected behavior that
>>
>>     1 = 0
>>
>> does not raise any error?  What does this mean?
>
> 1 is a valid (nullary) pattern, so yes. This pattern binding obviously fails.
>
>   Prelude> let x at 1 = 0
>   Prelude> x
>   *** Exception: <interactive>:2:5-11: Irrefutable pattern failed for pattern x at 1

If it can make things clearer, you can do something similar with any
constructor:

    > let Just x = Just 5 in x
    > let Just x = Nothing in x
*** Exception: <interactive>:19:5-20: Irrefutable pattern failed for
pattern Data.Maybe.Just x