Taming Errors with FunctionalKit

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

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

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

Taming Errors with FunctionalKit

by nkpart

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