Add Item Specifications

Item-Specifications.md

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+This page is devoted to concepts and designs for the structure of items in the TypeDispatcher and how they are specified.
+
+## Map-like object indexed by identifier and types
+
+In this concept, we think of the TypeDispatcher object `math` simply as a big map indexed by a string identifier and a list of types, with a handful of special properties:
+
+* The `math` object is set up so that you can write `math.foo(a, b, c)` to call `math['foo', [typeOf(a), typeOf(b), typeOf(c)]](a, b, c)`.
+* The stored entities are "dependent objects" -- they record at their creation time what other entries of the TypeDispatcher object they depend on, and if any of those change, they are regenerated as needed.
+* The keys and values of the `math` object can be specified in a pattern-matching style that supports/simulates generic functions
+
+How would this look in practice? Let's start by considering a `square` function that can operate on any type and depends on the `multiply` entry for that same type, and just calls that entry with both arguments set to the original single argument. We might specify this as:
+```
+export const square = (math, [T]) => {
+   const mul = math.get('multiply', [T, T])
+   return a => mul(a, a)
+}
+```
+Note that we should not write
+```
+export const square = (math, [T]) => {
+    return a => math.multiply(a, a)
+}
+```
+That's because in order to work as a dependency, the dependent items must be extracted within the body of the function that provides the final behavior/value for the item, not within that behavior.