Re: [CS3304_1381] variable storing value from function

Stephen Edwards (edwards@CS.VT.EDU)
Fri, 3 Mar 2000 12:11:19 -0500

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Message-ID:  <38BFF237.B3DAFD88@cs.vt.edu>
Date:         Fri, 3 Mar 2000 12:11:19 -0500
From: Stephen Edwards <edwards@CS.VT.EDU>
Subject:      Re: [CS3304_1381] variable storing value from function

> I've been practically banging my head on my desk over this one... someone
> please tell me what's wrong with this:

Stick to using "let" for introducing local names, and only use
"define" for creating top-level (global) names. The correct use
of "let" in your example looks like this:

    (define eval-line "blah")

    (define (function)
        (let ((line-value (eval-line)))
            (write-string line-value)
            #t
        )
    )

The structure of a "let" looks like:

    (let
        <declaration list>
        <expressions>
        .
        .
        .
    )

In a "named let", a single symbolic name precedes the <declaration
list>. The <declaration list> is a *single* list. Each element in the list
is itself another list representing a single declaration--it is a list
with two elements: the new variable name followed by the variable's value.
In general, that looks something like this.

    (let
        ( ; start of the declaration list
           (variable1 value1) ; first declaration
           (variable2 value2) ; second declaration
           ... ; more declarations
        ) ; end of declaration list
        (expression1) ; first "statement" in body of let
        ... ; more statements, as necessary
    )

In your example, there was only one local name being introduced,
so stripping out the comments and combining the lines a bit gives:

    (let ((line-value (eval-line)))
        (expresssion1)
        ...
    )

If you count the parentheses, you'll see how they match up to the
more expanded version shown above. My advice: if the more expanded
version is easier for you to read, then use it! There are no bonus
points or speed increases just because you write compact source
code :-).

                                -- Steve

--
Stephen Edwards            641 McBryde Hall          Dept. of Computer Science
e-mail      : edwards@cs.vt.edu           U.S. mail: Virginia Tech (VPI&SU)
office phone: (540)-231-5723                         Blacksburg, VA  24061-0106
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