A ''procedure'' is a [command] that is created by `[proc]`.

For example, `[info args]` expects its argument to be the name of a procedure.
If it is given the name of a command that is not a procedure, it will
`[return]` an [error].  `[info body]` also expects its argument to be the name
of a procedure.

----

Some programming languages draw a distinction between a '''procedure''' or
'''subroutine''', and a '''function'''.  When such a distinction is made, a
function is usually more similar to a mathematical function -- a map from its
inputs to a value.  In [Pascal], for example, a procedure does not return a
value, and can not be called from a point in code where a value is expected,
whereas a function returns a value and can be called from such a point.

In [Perl], a subroutine returns a value and can be used in both roles.
[Python] does the same, but chose the term "function" instead of "subroutine".
In both of those languages, as well as in most traditional imperative
languages, a '''statement''' serves as the framework for calling procedures and
functions, and manipulating their results.  Tcl unifies all three concepts into
one single concept:  The [command].  This is what makes [control
structure%|%control structures] in Tcl so flexible.  They cannot be implemented
as part of the syntax of a statement because Tcl has no such thing.  It only
has commands.

[Functional programming%|%Functional] languages also unify the three concepts,
"procedure", "function", and "statement" into the concept of a "function".
It's this common unification that makes functional programming feasible in Tcl.

The language of `[expr]` avoids the concept a "statement" by not providing any
syntax to allow evaluation of multiple expressions in a single call to
`[expr]`.  It also provides a functions, with syntax that is more similar to
standard mathematical notation.  `[expr]` functions are also surfaced as
commands in the [tcl::mathfunc] namespace, illusting that a [command] is the
more general concept.  Any Tcl command can be exposed as an `[expr]` function,
and such functions are not limited to the mathematical sense of mapping inputs
to a value, but retain the generic power of any Tcl command.



** See Also **

   [Pass by reference]:   
   
   [Implicit upvar]:   


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