taming errors with
FunctionalKit
Tuesday 24 March 2009

Error handling in Obj-C
‣ nil
‣ NSError
‣ @throw, and others?
Tuesday 24 March 2009

nil
Tuesday 24 March 2009

‣ MyObject *object = [dictionary
objectForKey:@”key”]; // Object might be nil.
‣ [object doSomething] => nil
Tuesday 24 March 2009

Pros
Easy to ignore
Safe messaging
Tuesday 24 March 2009

Cons
Easy to ignore
‘Safe’ messaging
Variadic functions (nil termination)
Not in the Type - When do you get nils?
You don’t have to think about it
No error message
Tuesday 24 March 2009

NSError
Tuesday 24 March 2009

‣ NSError *error;
‣ id val = [something withObject:object error:&error];
‣ if (!error) { ... } else { ... }
‣ Or just use val directly.
Tuesday 24 March 2009

Pros
Easy to ignore
Failure messages
Clear to the API consumer
Tuesday 24 March 2009

Cons
(Arguably) dirtier code
(Arguably) dirtier selectors
I don’t like output parameters
Mutable state
Tuesday 24 March 2009

So...
Tuesday 24 March 2009

Fancy Functional
Types
Tuesday 24 March 2009

nil → Option
Tuesday 24 March 2009

Option - A better nil
It fails when messaged
explicit handling
Tuesday 24 March 2009

single value array
Tuesday 24 March 2009

Some value
Tuesday 24 March 2009

Or none.
Tuesday 24 March 2009

Getting an Option
‣ + (FKOption *)fromNil:(id)maybeNil;
‣ + (FKOption *)fromNil:(id)maybeNil ofType:(Class)cls;
‣ + (FKOption *)some:(id)value;
‣ + (FKOption *)none;
Tuesday 24 March 2009

Using an Option
‣ - @property (readonly) BOOL isSome;
‣ - @property (readonly) BOOL isNone;
‣ - @property (readonly) id some;
‣ - (id)orSome:(id)some;
Tuesday 24 March 2009

The Code
Tuesday 24 March 2009

(value|NSError) →
Either
Tuesday 24 March 2009

Getting an Either
‣ + (FKEither *)eitherWithRight:(id)value;
‣ + (FKEither *)eitherWithLeft:(id)value;
Tuesday 24 March 2009

Using an Either
‣ @property (readonly) FKRightProjection *right;
‣ @property (readonly) FKLeftProjection *left;
‣ @property (readonly) BOOL isLeft;
‣ @property (readonly) BOOL isRight;
Tuesday 24 March 2009

Using a Left/Right
‣ @property (readonly) id *value;
‣ @property (readonly) id valueOrOther:(id)value;
‣ - (id)valueOrFailWithMessage:(NSString *)message;
Tuesday 24 March 2009

‘Wrong’ on Left
‘Right’ on Right
Tuesday 24 March 2009

The Code
Tuesday 24 March 2009

Is it better?
Tuesday 24 March 2009

Still have to extract.
Can still be noisy.
Tuesday 24 March 2009

List of As
map:: (a→b)→[a]→[b]
List of Bs
Function from A to B
Tuesday 24 March 2009

Mapping with Option
‣ - (FKOption *)mapWithSelector:(SEL)selector;
‣ - (FKOption *)mapWithSelector:(SEL)selector onObject:
(id)object;
Tuesday 24 March 2009

The Code
Tuesday 24 March 2009

Mapping with Either
‣ - (FKEither *)mapWithSelector:(SEL)selector;
‣ - (FKEither *)mapWithSelector:(SEL)selector onObject:
(id)object;
Tuesday 24 March 2009

The Code
Tuesday 24 March 2009

The Code
OR
OR
Tuesday 24 March 2009

By the way...
Tuesday 24 March 2009

Option is a Monad.
Tuesday 24 March 2009

Either is a Monad
too.
Tuesday 24 March 2009

FunctionalKit
http://github.com/mogeneration/functionalkit
Tuesday 24 March 2009

FunctionalKit
‣ FKOption
‣ FKEither
‣ FKFunction (a->b)
‣ FKEffect (a -> Nothing)
‣ FKP*
‣ FKNewtype
‣ FKMacros
Tuesday 24 March 2009

resources
http://github.com/mogeneration/
functionalkit
http://www.qconlondon.com/london-2009/
presentation/Null+References:+The+Billion
+Dollar+Mistake
Tuesday 24 March 2009