Daddy, how does a computer work

Arjen Markus (25 june 2003) My elder daughter (6 and a half now, and very keen to learn new things) regularly asks me to explain how the magical device called computer works. So, invariably I tell her that you tell the computer to do this and that. Wrong metaphor! I then have to explain that a computer can not really hear and understand me. And that usually ends the explanation's main course.

So, why not use Tcl and the package I use for Geometrical constructions? This is the result - okay, I immediately admit that the drawing of the archetypical desktop computer is very boring - but simply run it and see the "story" unfold.

# howcomputer.tcl --
#    A small story: how does a computer work - for a 6-years' old child
#    Source into "constructions.tcl"
#
namespace import ::Constructions::*
display
mode cartesian
moveto 0 5.5
textcolour black
textfont "times 18"
draw text "How does a computer work?"

#
# Draw a "computer"
#
moveto 2 2
draw line   2   2    2   5
draw line   2   5    5   5
draw line   5   5    5   2
draw line   5   2    2   2
draw line   2   2  1.8   2
draw line 1.8   2  1.8 1.5 
draw line 1.8 1.5  5.2 1.5
draw line 5.2 1.5  5.2   2
draw line 5.2   2    5   2
moveto 3.5 4.5
textfont "times 10"
draw text "Type command: _"
#
# Show the user typing a command
#
moveto -4 1
draw text "I type:"
textcolour blue
moveto -4 0
set string {}
foreach letter [split "print \"Hello!\"" {}] {
    append string $letter
    set tobj [draw text $string]
    erase $tobj
}
draw text $string

#
# The computer reacts "instantaneously"
#
moveto 3.5 4
draw text $string
moveto -2 -1
draw text "The computer answers:"
textfont "helvetica 18 bold"
moveto 0 -2
draw text "Hello!"

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