Using a Function to emulate F# Match in C#

Question

So this was inspired by: this question But doesn't actually answer it: What do you think of:

public class Case<TRes>
{
    public Case(bool condition, Func<TRes> result)
    {
        Condition = condition;
        Result = result;
    }
    public bool Condition {get; private set;}
    public Func<TRes> Result { get; private set; }

    public static Case<TRes> Default(Func<TRes> value)
    {
        return new Case<TRes>(true, value);
    }
}

public static class Logic
{
    public static T Match<T> (params Case<T>[] cases)
    {
        return cases.First(c => c.Condition).Result();
    }
}

Which you could invoke:

int i = Logic.Match
(
    new Case<int>(IsCloudy(skyImage), () =>
    {
        ChangeWeatherIcon(Icons.Clouds);
        return 13;
    }),
    new Case<int>(a < b, () => a),
    Case<int>.Default(() => {throw new Exception(); })
);

it's purpose isn't quiet to emulate F#'s Match. (Because F# match doens't actually take conditions, it is a stuctural/pattern match) So really its kind of like the trinary operator, meets the nonOrdinal Select statment.

Anyone got any idea's on how to make it work cleaner? Maybe getting a way to get rid of the new's and the brackets

If we swap Func(T) for T, we get rid of the brackets etc, at the expense of nolonger being able to specify things to be done. (so it would be more like the trinary operator than a If) — Frames Catherine White
– Frames Catherine White, Commented Oct 2, 2011 at 14:04
You might be interested in reading this blog post which has been doing something similar. It uses expressions and analyzes them to perform the matching. It's more or less the approach I had in mind. Take a look. — Jeff Mercado
– Jeff Mercado, Commented Oct 3, 2011 at 18:46

Rafe · Accepted Answer · 2011-10-03 03:01:15Z

1

To be honest, it's pretty much impossible to do elegant pattern matching in a language that doesn't support it (e.g., C#). The problem with what you've written there is it's more complicated than the equivalent if-then-else chain:

if (IsCloudy(skyImage)) {
    ChangeWeatherIcon(Icons.Clouds);
    return 13;
}
if (a < b) {
    throw new Exception();
}

Now, I would dearly love to see some later version of C# adopt algebraic data types (a la F#) with pattern matching, but until then I very much doubt you'll be able to do better than if-then-else chains.

answered Oct 3, 2011 at 3:01

Rafe

1,3589 silver badges12 bronze badges

\$\begingroup\$ I'm not sure if you realise, but your if/else change is not (at all) equivelent. Equivelent is: int i; if (IsCloudy(skyImage) { ChangeWeatherIcon(Icons.Clouds); i = 13; } else if (a < b) i= a; else throw new Exception(); It would be alot nicer spread over multiple lines \$\endgroup\$

Frames Catherine White
– Frames Catherine White

2011-10-03 11:15:30 +00:00
Commented Oct 3, 2011 at 11:15
1

\$\begingroup\$ You're right, but - meh - that's beside my point, which is that in attempting to do something more elegant than if-then-elses, you've made your code harder to read. \$\endgroup\$

Rafe
– Rafe

2011-10-03 23:12:17 +00:00
Commented Oct 3, 2011 at 23:12

Add a comment |

Mauricio Scheffer · Accepted Answer · 2014-02-04 16:47:00Z

I generally agree with Rafe: it's not worth trying to do this in C# for the general case. Using expressions, etc, simply don't cut it, IMHO pattern matching is something that must be built into the language (or use a malleable language like Lisp). The main point of pattern matching is that it's exhaustive (or at least you get a warning if it's not), and therefore total. If you can't guarantee that, then you don't gain anything.

However, I do think it's fruitful to use pattern matching for specific cases. For example, in FSharpx we have pattern matching for C# on options, lists and choices (i.e. anonymous discriminated unions).

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