Option type about c5

[sestoft]

On Mon, 27 Jun 2011, Qbit86 wrote:

Why do you use deprecated out- and ref-parameters to return values?

This is a good idiom in imperative code. In functional code it
obviously is a bad idea.

Peter

[qbit86]

This is a good idiom in imperative code. In functional code it obviously is a bad idea.

But my C#-code is as functional as possible :) And out/ref + assignments are too imperative :(

You say: «The design has been influenced by the collection libraries for Java and Smalltalk and the published critique of these.» (http://www.itu.dk/research/c5/latest/ITU-TR-2006-76.pdf) It's probably worth looking at collection libraries in mixed-paradigm languages as F# (http://msdn.microsoft.com/en-us/library/ee353413.aspx).
Exception-free search operations always return option as zero-or-one-element collection (in Haskell they return Maybe monad):
http://msdn.microsoft.com/en-us/library/ee353739.aspx
http://msdn.microsoft.com/en-us/library/ee353786.aspx
etc.
It's very handy (even if one cannot use pattern matching in C#): you don't need to divide initialization into declaration and assignment; your locals are immutable, their values are predictable. Compare current usage with suggested:
// 1)
K key = ...
K actualKey = key;
V value;
if (dictionary.Find(ref actualKey, out value)) { /* Use actualKey, value. / }
// 2)
K key = ...
KeyValueOption<K, V> result = dictionary.Find(key);
if (result.HasValue) { / Use result.Key, result.Value. */ }

In C++ STL we've got similar pattern: get result handle (iterator—Option), compare it with sentinel (v.end()—Option.None), retrieve it from handle (dereferense—Option.Value).

Class Option can be implemented as in Elevate Library (http://elevate.codeplex.com/), for eaxmple.

[ondfisk]

Agreed the Option type of F# is a very neat solution.

However, C5 also tries to mimic the standard C# interfaces and programming styles and here the so called Try Get pattern is most often used. Cf. Krzysztof Cwalina, Brad Abrams: Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries (http://www.amazon.com/Framework-Design-Guidelines-Conventions-Development/dp/0321246756).

C5 is currently littered with Try Get and it would be an enormous undertaking to convert these into Option type style - and also be a major breaking change it the old Try Get methods were not to remain in the library.