Context

A (Context a) associates Text labels with values of type a. Each Text label can correspond to multiple values of type a

The Context is used for type-checking when (a = Expr X)

• You create a Context using empty and insert
• You transform a Context using fmap
• You consume a Context using lookup and toList

The difference between a Context and a Map is that a Context lets you have multiple ordered occurrences of the same key and you can query for the nth occurrence of a given key.

An empty context with no key-value pairs

Add a key-value pair to the Context

Look up a key by name and index

lookup _ _         empty  = Nothing
lookup k 0 (insert k v c) = Just v
lookup k n (insert k v c) = lookup k (n - 1) c
lookup k n (insert j v c) = lookup k  n      c  -- k /= j

Return all key-value associations as a list

toList           empty  = []
toList (insert k v ctx) = (k, v) : toList ctx