Staticly scoped environments

(from [2a3f31e387e7b41c1856ff79a47ad5bfebdea5cc]) CSPM environments

An environment is the symbol table that hold CSPM definitions. They provide a standard mapping between identifiers and the expressions that define them. Environments can also have an optional parent, which allows nested scoping; if an identifier is not found in the current environment, we then proceed to look for it in the containing environment.

This patch introduces the basic API for creating and using environments, but does not extend the evaluation rules to use them.

Lighthouse: [#11 Staticly scoped environments] http://github.com/dcreager/hst/c...

(from [37739ef6e867fa53cb039c707bf15d4e1affd1ba]) Flattening the Expression type

Before, we had separate datatypes for each “class” of expression; for instance, a Number type for any expression that always evaluates to a number. The Expression type was then defined as a sum type across all of these subtypes.

This patch flattens things so that there's only a single Expression type. This simplifies things by removing a level of indirection in the type definitions, and also will make it easier to define “bound expressions” as we implement static-scoped environments.

Lighthouse: [#11 Staticly scoped environments] http://github.com/dcreager/hst/c...

(from [260a78539f5808c2553f028caf28fc47e57ddb0c]) Let bindings

This patch introduces evaluation rules for let bindings and variable references. Currently, this is the only way to define the value for a variable.

Lighthouse: [#11 Staticly scoped environments] http://github.com/dcreager/hst/c...