[Haskell-cafe] Pattern Matching

[Andrew Coppin]

Colin Paul Adams wrote:
> If I had a dollar for every time I've written something like
>
>     Andrew>  case msg of eVENT_QUIT -> ...  eVENT_POST -> ...
>     Andrew> eVENT_RESIZE -> ...
>
>     Andrew> and spent an hour trying to figure out why the messages
>     Andrew> aren't being processed right... ;-)
>
> So why aren't they?
>   

Because what I *should* have written is

  case msg of
    _ | msg == eVENT_QUIT -> ...
       | msg == eVENT_POST -> ...
       | msg == eVENT_RESIZE -> ...

which is something quite different.

(And, entertainingly, because the incorrect version is perfectly valid 
source code, no compiler errors or warnings...)