String split zero copy

text/0000-string-split-zero-copy.md

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+- Feature Name: String split zero-copy
+- Start Date: 2016-10-18
+- RFC PR:
+- Pony Issue:
+
+# Summary
+
+The string splitting method should work on an immutable copy and provide a "zero-copy" functionality.
+
+# Motivation
+
+This behavior is preferable because you get to decide whether to make zero or one copy of the complete string. It's easy to obtain an immutable copy of a string so usability is preserved.
+
+# Detailed design
+
+The splitting method will be revised to operate on the immutable string only, returning immutable zero-copy reference strings to the same memory allocation.
+
+The previous method signature was:
+```pony
+fun split(delim: String = " \t\v\f\r\n", n: USize = 0): Array[String] iso^ =>
+```
+This would be changed to:
+```pony
+fun val split(delim: String = " \t\v\f\r\n", n: USize = 0): Array[String] val^ =>
+```
+
+Typical usage:
+```pony
+let words = "Hello world".split()
+let s = "1,2,3".clone()
+let numbers = s.split(",")
+```
+The latter example relies on auto-recover to make the cloned ``String iso`` immutable.
+
+# How We Teach This
+
+This API is a good candidate for inclusion in the tutorial. It's a common operation and it highlights how reference capabilities fit naturally in library design.
+
+# How We Test This
+
+The existing tests will be adapted.
+
+#### Drawbacks ####
+
+This breaks existing code.
+
+# Alternatives
+
+Not considered.
+
+# Unresolved questions
+
+No open questions.