Add raw notes for Mar 28, 2018

meetings/2018/LDM-2018-03-28.md

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+# C# Language Design Notes for Mar 28, 2018
+
+***Warning: These are raw notes, and still need to be cleaned up. Read at your own peril!***
+
+Ranges
+
+# Shipping a prototype
+
+We would like to ship a prototype of ranges, in order to get feedback and settle the remaining decisions.
+
+We would need to offer the Range type, maybe as a NuGet.
+
+But it would also be nice to offer range-enhanced other types, like `Span<T>` and maybe arrays.
+
+Maybe the compiler can fake out special indexers in the prototype. Or we also add extension indexers.
+
+Tying it to revs of .NET Core previews may be too sluggish, and fraught with dependencies.
+
+Or we create a wrapper type for Spans with the extra behavior, and smooth over the edges as best we can (implicit conversions etc)
+
+## Conclusion
+
+Let's do:
+
+1. A temporary "language feature" that's extension indexers based on method names
+2. Range support in the language
+3. A preview NuGet package with `Range`, associated types and "extension indexers" on known types
+
+
+# Which feature?
+
+We have two options:
+
+1. Do like Python: just live with "0 from end" not being expressible as a number; it's a special case
+2. Add Index and ^
+
+The first one is the more restrictive, and also the cheaper one (from both work and number of abstractions). This will help us understand whether people need the extra.
+
+But not supporting "from end" is too restrictive; we have very good reason to believe people need that.
+
+
+# Nested stackalloc
+
+Issue #1412, trivial to implement in 8.0, where the stack-spilling machinery is there anyway for pattern matching.
+
+
+# Exhaustiveness in switch expressions
+
+Should non-exhaustiveness be an error or a warning? If warning, what should happen at runtime?
+
+For now, it's a warning, and if you get there we throw a new exception type for this.
+