[Proposal] Better handling of multiple comparisons with the same variable (`a > min && a < max`)

Problem

Consider these very common code patterns:

if (a > min && a < max) ...
if (value == Enum.Value1 || value === Enum.Value2) ...

I think it is worth considering how verbosity here can be reduced – especially since it could also give a performance benefit for cases like

if (Slow() > min && Slow() < max) ...

Potential solutions

A. Ternary comparisons

For <, >, >=, and <= one solution can be Python-like min < a < max.

Pros:

1. Good readability
2. Performance benefit since a can be evaluated exactly once

Cons:

1. Does not handle a == x || a == y.

B. Range comparisons

Example:

if (a in (min...max)) ...
if (value in (Enum.Value1, Enum.Value2)) ...
if (x in (1,2,3..7)) ...

Pros:

1. Alright readability
2. Covers many scenarios
3. Can be easily adapted for future switch (case 1..3:).

Cons:

1. Not clear whether the range is inclusive or exclusive
2. Too much new syntax for a simple case?

C. Quantum superpositions

Example:

if (value == any(Enum.Value1, Enum.Value2)) ...

This is similar to Perl’s Quantum Superpositions and something that was already implemented in Microsoft Research’s Cω.

Pros:

1. Good readability
2. Can be adapted for future switch

Cons:

1. In practice only covers equality (x > any(1,2) is possible, but useless in most scenarios). Could be combined with solution A for a limited comparison support.

[HaloFour]

It was mentioned on CodePlex that functionality like this could be pretty easily accomplished through the pattern matching proposals and extension methods.

e.g.:

using static Some.Namespace.RangePatternMatcher;

if (x is between(5, 10)) { } // range comparison
if (x is any(1, 2, 3, 4, 10)) { } // set comparison

[alrz]

@gafter It was mentioned in #7632. The pattern counterpart is something that one would expect.