** Summary **

Some of those [Learning Tcl%|%learning] to program in [Tcl] are also new to
programming in general.  This page approaches Tcl in that light.

This page is just getting started so feel free to pitch in!


** Description **

There are, of course many resources out there in the world, that teach
beginners the art of computer programming.  One of the more well-known is
''[BOOK Structure and Interpretation of Computer Programs%|%The Structure and
Interpretation of Computer Programs]'', but it uses [LISP] rather than Tcl.
Tcl is a fantastic first language for someone interested in getting into
computer programming, and this page teaches Tcl obliquely by teaching the
general art of programming, using Tcl as a resource to illustrate the concepts.


** Values and side effects **

Some commands return useful values, some have side effects, and some do both.


The value of `[[[puts]]` is invariant.  It always returns the empty string, so
it's never useful:

======
set a [puts hello]
======

Therefore, `[[[puts]]` has a side effect, but no useful value.  

`[[[set]]` has a side effect.  It creates a new variable and assigns a value to
it.  `[[[set]]` also returns the value that it assigned to the variable, which
can be useful sometimes.  To set two variables to the same value, one could
write:

======
set b [set a hello]
======

Therefore, `[[[set]]` has a side effect and also has a useful value.

`[[[string length]]` is an example of a command that doesn't have any side
effects but does have a useful value.  It changes nothing in the world of a Tcl
script.  It doesn't create or delete any commands or variables, and it doesn't
write any data to any channels.  However, it returns a value that tells us
something we might want to know:

======
string length hello
======