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Re: Repeat syntax
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From: |
Taylan Ulrich Bayırlı/Kammer |
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Subject: |
Re: Repeat syntax |
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Date: |
Sat, 30 Dec 2017 14:18:35 +0100 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Christopher Howard <address@hidden> writes:
> Hi list, I want to have a function
>
> (repeat n exp exp* ...)
>
> That calls the expressions n times for side effect, like for-each, but
> without the bother of dealing with a list. It seems like somebody else
> must have thought of this before, but I couldn't find the equivalent
> procedure. After reading 6.10.2 I came up with this
>
> (define-syntax repeat
> (syntax-rules (repeat)
> ((_ n exp exp* ...)
> '(unless (<= n 0)
> exp
> exp*
> ...
> (repeat (- n 1) exp exp* ...)))))
>
> Which doesn't work I think because repeat gets expanded infinitely many
> times. I was pondering other ways to do this, but they all seem to end
> in either infinite expansion, or an important variable getting
> overshadowed. So, could somebody point me in the right direction?
The 'do' syntax, although difficult to get used to, can do this:
(do ((times 10 (- times 1)))
((zero? times))
<body>)
Here's the specification of 'do':
(do ((<variable [id]> <init-val [expr]> <successor-val [expr]>)
...)
(<termination-condition [expr]> <finisher [expr]> ...)
<command [expr]>
...)
The metasyntax "x ..." means ZERO or more occurrences of x.
Evaluation rules:
1. The <variable> and <init-val> pairs are bound as if in a 'let'.
2. The <termination-condition> is evaluated:
A. If true, the <finisher>s are evaluated in order, the last result
being the result of the 'do' expression.
B. If false:
a. The <command>s are evaluated in order.
b. The <variable> and <successor-val> pairs are bound as if in a
'let' (the previous bindings of <variable>s are available).
c. Go to step 2.
Taylan
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