Add RFC for SortBy

text/0000-sort-by.md

@@ -0,0 +1,42 @@
+- Feature Name: sort-by
+- Start Date: 2020-04-27
+- RFC PR: (leave this empty)
+- Pony Issue: (leave this empty)
+
+# Summary
+
+Add a `SortBy` primitive to `collections` package.
+
+# Motivation
+
+The generic parameter of `Sort` requires the implementation of the `Comparable` interface， if the existing type does not implement `Comparable` interface will be helpless, `SortBy` allows any type, it solves this problem by injecting a lambda.
+
+# Detailed design
+
+- `SortBy` has the same interface as `Sort`. 
+- `SortBy` uses a `lambda` as the hash evaluation method instead of `interface Comparable`.
+
+For example:
+
+```pony
+use "collections"
+
+actor Main
+  new create(env:Env) =>
+    let array = [ "aa"; "aaa"; "a" ]
+    SortBy[String](array, {(x: String): U64 => x.size().u64() })
+    for e in array.values() do
+      env.out.print(e) // prints "a \n aa \n aaa"
+    end
+```
+
+# How We Test This
+
+All test cases for `Sort` are also valid for `SortBy`. Its design and implementation are already complete and working.
+Source : https://github.com/damon-kwok/pony-shoe/blob/master/sort_by.pony
+
+# Drawbacks
+
+- The user must ensure that the lambda evaluation function is valid.
+- Compared to Sort's compile-time check, `SortBy` has a little runtime overhead.
+