Create FS-1094-block.md

RFCs/FS-1094-block.md

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+# F# RFC FS-1093 - Block for ImmutableArray
+
+The design suggestion [A normative immutable array type](https://github.com/fsharp/fslang-suggestions/issues/619) has been marked "approved in principle".
+
+This RFC covers the detailed proposal for this suggestion.
+
+- [ ] Implementation TBD
+- [ ] Design Review Meeting(s) with @dsyme and others invitees
+- [ ] Discussion TBD
+
+# Summary
+
+A new collection type `'T block` will be added to FSharp.Core, and FSharp.Core will now be dependent on System.Collections.Immutable
+
+# Motivation
+
+Prior to this RFCS F# programming has no strong opinion on an immutable array data structure in regular F# coding.
+There are lots of use cases for this and it is easy enough to implement efficiently.
+
+Fable and the Elmish design patterns are popularizing the use of immutable data for important model descriptions
+more and more and we should be helping improve the situation for that kind of programming.
+
+The main question is to how to make on immutable array type normative in F# coding
+
+We have decided it is acceptable for FSHarp.Core to take a dependency on `System.Collections.Immutable`. We still want an FSharp.Core
+module making it look and feel like a normal F# collection.
+
+
+# Detailed design
+
+```fsharp
+namespace FSharp.Collections
+
+...
+
+type 'T block = System.Collections.Immutable.ImmutableArray<'T>
+
+[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
+[<RequireQualifiedAccess>]
+module Block =
+
+    let append(l1 : 'T block) (l2 : 'T block) = l1.AddRange(l2)
+
+    let empty<'T> = block<'T>.Empty
+    ...
+```
+
+etc. with the standard interface.
+
+### Structural equality, hashing and comparison
+
+`System.Collections.Immutable.ImmutableArray` implements `IStructuralEquality` and `IStructuralComparison`, so
+this means F# strucural equality, hashing and comparison semantics are respected "all the way down".
+
+TBD: examples and test cases
+
+
+# Drawbacks
+
+* `FSharp.Core` has a dependency on `System.Collections.Immutable.dll` and any transitive dependencies.  
+
+
+# Alternatives
+
+- Don't do this and maintain status quo
+
+- Make a separate package `FSharp.Cllections.Block.dll`
+
+- Use a different name
+
+# Compatibility
+
+This is not a breaking change. The elaboration of existing code that passes type checking is not changed.
+
+This doesn't extend the F# metadata format.
+
+# Unresolved questions
+
+* Full API design