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# 1. Jump Start

Before giving an overview of Guile, I present some simple commands and programs that you can type to get going immediately.

Start by invoking the Guile interpreter. Usually you do this by just
typing `guile`

. Then type (or paste) the following expressions at
the prompt; the interpreter's response is preceded (in this manual) by
⇒.

<shell-prompt> guile |

(+ 20 35) ⇒ 55 (define (recursive-factorial n) (if (zero? n) 1 (* n (recursive-factorial (- n 1))))) (recursive-factorial 5) ⇒ 120 (quit) |

In this example we did some simple arithmetic `(+ 20 35)`

and got
the answer `55`

. Then we coded the classic (and rather wasteful)
factorial algorithm and computed the factorial of `55`

. Finally we
quit with `(quit)`

.

We can find out about some of Scheme's nice features by asking for the
factorial of some big number, say `500`

. On some systems the
correct answer will be returned (I do not indicate calling and leaving
the guile session anymore).

(recursive-factorial 500) ⇒ 1220136825991110068701238785423046926253574342803192842192413588 3858453731538819976054964475022032818630136164771482035841633787 2207817720048078520515932928547790757193933060377296085908627042 9174547882424912726344305670173270769461062802310452644218878789 4657547771498634943677810376442740338273653974713864778784954384 8959553753799042324106127132698432774571554630997720278101456108 1188373709531016356324432987029563896628911658974769572087926928 8712817800702651745077684107196243903943225364226052349458501299 1857150124870696156814162535905669342381300885624924689156412677 5654481886506593847951775360894005745238940335798476363944905313 0623237490664450488246650759467358620746379251842004593696929810 2226397195259719094521782333175693458150855233282076282002340262 6907898342451712006207714640979456116127629145951237229913340169 5523638509428855920187274337951730145863575708283557801587354327 6888868012039988238470215146760544540766353598417443048012893831 3896881639487469658817504506926365338175055478128640000000000000 0000000000000000000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000 |

The result is an example of Scheme's *bignumbers*. However, there
are operating environments that provide (by default) too little stack
space. They will instead produce an error message like this:

(recursive-factorial 500) -| ERROR: Stack overflow ABORT: (stack-overflow) |

Rather than enlarging the system's stack, we can implement the algorithm
such that it does not consume increasing stack space. This is called a
*tail recursive* implementation. The following definition is tail
recursive and so should work on all systems.

(define (tail-recursive-factorial n) (define (loop k l) (if (zero? k) l (loop (- k 1) (* k l)))) (loop n 1)) (tail-recursive-factorial 500) ⇒ 1220136825991110068701238785423046926253574342803192842192413588 ;; ... skipped |

This is the most basic use of Guile: a simple Scheme interpreter. In
the rest of this tutorial I will show you how Guile has many facets: it
is also an *extensible* interpreter (to which many features can be
easilly added) and an *embeddable* interpreter (which can be
invoked from your C programs).

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