Use static typing where available

rfc-216-use-static-typing-where-available.md

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+# Use static typing where available
+
+## Summary
+
+Static typing can help prevent programming mistakes and bugs by making sure methods are only returning, or being passed,
+the types of data they expect. Since static typing is available in some form in all of our supported languages –
+optionally in Ruby, Python and PHP, in JavaScript via the use of TypeScript, and inherently in the language in Kotlin
+and C# – we should make use of static typing where available.
+
+## Problem
+
+In programming, every piece of data has a type. For example, whole numbers are integers, and a series of characters is a
+string. Generally speaking any method can only be accept some of these types - for example it doesn't make sense to
+multiply two strings, or to capitalise an integer.
+
+dxw predominantly programs in what are known as dynamically typed languages, where the types of data only become
+relevant when they are used at run-time. This means situations can arise where an unexpected type of data can be passed
+into an argument, for example through uncaught errors or user input, which can cause unexpected behaviour or outright
+failures.
+
+By making use of static typing we can explicitly declare which types of data methods are expected to return, and which
+types of data they accept as arguments. Type checking (at compile-time for compiled languages, and via separate tooling
+for uncompiled) can then make sure that no situation arises in the code where data is either returned or passed in an
+unsupported type.
+
+Although this doesn't guarantee bug programming, and does not replace the need for testing or peer review, it does offer
+guard rails against some common mistakes and encourages consideration of exactly what types of data any piece of code
+will be expected to handle.
+
+## Proposal
+
+Where a language either includes support for static typing or where tooling exists to ensure it, programs we write
+SHOULD make use of static typing.
+
+Where static typing is used, any automated test pipeline MUST make sure that types are checked.
+
+If an existing project is in a language which supports optional or partial typing, you MAY only type parts of the
+codebase as they are modified.
+
+## Next steps
+
+- [ ] Update language templates to include type checking as part of their test process