[Haskell-cafe] Re: not possible with monad transformers ?

[Marcin 'Qrczak' Kowalczyk]

Jules Bean <jules at jellybean.co.uk> writes:

>> When writing a compiler, it makes sense to collect errors as by the
>> writer monad, and not abort anything - producing dummy values instead
>> (except perhaps some fatal errors when it's inconvenient).
>
> Or you could use the monad:
>
> data Perhaps a = Success a | Failure a [Error]

> This is just a special case of the writer monad, I think.

It's the same as the writer monad, with redundant encoding of the case
when there are no errors.

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/