Nim/doc/manual/definitions.txt

Definitions
===========

A Nim program specifies a computation that acts on a memory consisting of
components called `locations`:idx:. A variable is basically a name for a
location. Each variable and location is of a certain `type`:idx:. The
variable's type is called `static type`:idx:, the location's type is called
`dynamic type`:idx:. If the static type is not the same as the dynamic type,
it is a super-type or subtype of the dynamic type.

An `identifier`:idx: is a symbol declared as a name for a variable, type,
procedure, etc. The region of the program over which a declaration applies is
called the `scope`:idx: of the declaration. Scopes can be nested. The meaning
of an identifier is determined by the smallest enclosing scope in which the
identifier is declared unless overloading resolution rules suggest otherwise.

An expression specifies a computation that produces a value or location.
Expressions that produce locations are called `l-values`:idx:. An l-value
can denote either a location or the value the location contains, depending on
the context. Expressions whose values can be determined statically are called
`constant expressions`:idx:; they are never l-values.

A `static error`:idx: is an error that the implementation detects before
program execution. Unless explicitly classified, an error is a static error.

A `checked runtime error`:idx: is an error that the implementation detects
and reports at runtime. The method for reporting such errors is via *raising
exceptions* or *dying with a fatal error*. However, the implementation
provides a means to disable these runtime checks. See the section pragmas_
for details.

Whether a checked runtime error results in an exception or in a fatal error at
runtime is implementation specific. Thus the following program is always
invalid:

.. code-block:: nim
  var a: array[0..1, char]
  let i = 5
  try:
    a[i] = 'N'
  except IndexError:
    echo "invalid index"

An `unchecked runtime error`:idx: is an error that is not guaranteed to be
detected, and can cause the subsequent behavior of the computation to
be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx:
language features are used.