CODING CONVENTIONS
==================

* Naming conventions

type names:        FOO_BAR
constructor names: FooBar
local/field names: fooBar

* Non-exhaustive matches

During implementation, non-exhaustive match warnings are fine (they
help us keep track of what hasn't been implemented). But completed
code should not have non-exhaustive matches.

If a particular case should never happen, the right-hand side of the
match case should raise a Fail exception with a string indicating what
invariant was violated.

* Explicit type annotations

Top-level functions must be annotated explicitly.

* Line widths

Lines must not exceed 100 columns.

* Indentation

[TODO]

* Tabs

Hard tabs are disallowed.

* Internal errors

The exception HostError represents *internal evaluator errors*, that
is, errors that are not the fault of the user but instead a problem
with the evaluator.

There is a global invariant that for any ES4 program, HostError can
never be raised. If a HostError is raised, this invariant has been
violated, i.e. there's an internal inconsistency in the evaluator.

HostError should be the only exception used to represent internal
evaluator errors.

HostError is defined in logerr.sml.