[Haskell-beginners] Guard in class def

Henry Lockyer henry.lockyer at ntlworld.com
Sun Jan 8 15:40:00 CET 2012

OK - thanks (both)!  

On 8 Jan 2012, at 11:44, Chaddaï Fouché wrote:

> On Sun, Jan 8, 2012 at 10:29 AM, Henry Lockyer
> <henry.lockyer at ntlworld.com> wrote:
>> Hi,
>> I was just looking through the 'Monad Transformers' chapter in Real World Haskell. They are using the "reader" monad to illustrate the transformer structure  but I fell off at the first bend when I saw the following:
>> class (Monad m) => MonadReader r m | m -> r where
>>  ask     :: m r
>>  local  :: (r -> r) -> m a -> m a
>> Could someone explain the use of a guard here / how to read this with the "| m -> r"  ?
>> I haven't come across this usage before (as far as I have noticed :-) and the meaning hasn't jumped out at me yet...
> It's not a "guard", it's a functional dependency : "m -> r" tells the
> compiler that for any m, r is uniquely determined. That is if you have
> the monad "ReaderT Int Identity" for m, it is clear that "Int" is the
> only possibility for r.
> This is necessary for type inference to process in more case (often
> the type of the monad is fixed by the context, but not the type of r,
> thanks to the FD, the compiler can confidently use the instance it
> found for m).
> Note that this solution never really satisfied the Haskell
> users/developers since it is more from the logic paradigm (like
> Prolog) than the functional paradigm, so since two or three years now,
> this tends to be replaced by a new solution, more functional in style,
> called type family, which would read like that :
>> class (Monad m) => MonadReader m where
>>   type RC :: * -> * -- Reader content
>>   ask     :: m (RC m)
>>   local  :: (RC m -> RC m) -> m a -> m a
> where RC is like a "function on type".
> -- 
> Jedaï

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