newbie: read, load and eval

newbie: read, load and eval

[dale]

This seems like the simplest thing which guile ought to be able to do
in a snap, but I can't for the life of me work out how to do it.

I have a file called a.data which contains some data as a scheme
expression, such as

'("This" "is" "a" "test")

and I want to use these data in a program, so I want to write a line
like

(display (load "a.data"))

but it doesn't work; nor does

(with-input-from-file "a.data" (lambda () (display (read))))


What is the way to do this?


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Re: newbie: read, load and eval

[Mike Gran]

Let's say you had a file "a.data" that contained

("This" "is" "a" "test")

Note that there is no initial quote.

One way would be 

(display (read (open-input-file "a.data")))

but this doesn't explicity close the file "a.data" right after the call, which might be an issue if you are opening a lot of files.

One step up could be

(call-with-input-file "a.data"  (lambda (p) (display (read p))))

This will explicity close the file after it is read.

Note that this specifically reads exactly one s-expression from the top of the file, as you requested.  It might be simpler to have an "a.data" that contained a bunch of (define) statements, like so

(define a '("This" "is" "a" "test"))

Then you could (load "a.data") that file, and the defined variables would be available.

Hope this helps,

Mike Gran

----- Original Message ----
From: "dale@dmellor.dabsol.co.uk" <dale@dmellor.dabsol.co.uk>
To: guile-user@gnu.org
Sent: Saturday, October 21, 2006 11:48:04 PM
Subject: newbie: read, load and eval

This seems like the simplest thing which guile ought to be able to do
in a snap, but I can't for the life of me work out how to do it.

I have a file called a.data which contains some data as a scheme
expression, such as

'("This" "is" "a" "test")

and I want to use these data in a program, so I want to write a line
like

(display (load "a.data"))

but it doesn't work; nor does

(with-input-from-file "a.data" (lambda () (display (read))))


What is the way to do this?


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Re: newbie: read, load and eval

[dale]

Mike Gran writes:
>> 
>> Let's say you had a file "a.data" that contained
>> 
>> ("This" "is" "a" "test")
>> 
>> Note that there is no initial quote.
>> 
>> ...
>> 
>> One step up could be
>> 
>> (call-with-input-file "a.data"  (lambda (p) (display (read p))))

   Perfect!  I was basically making a mistake with the initial quote.



>> Note that this specifically reads exactly one s-expression from the
>> top of the file, as you requested.  It might be simpler to have an
>> "a.data" that contained a bunch of (define) statements, like so
>> 
>> (define a '("This" "is" "a" "test"))
>> 
>> Then you could (load "a.data") that file, and the defined variables would be available.

    Yuk!  That would mean that I have two independent pieces of code
which need to agree with each other to work properly, opening the door
for bugs... it also would pollute the top-level environment.  I like
your first answer!



>> Hope this helps,

    Sure does! Thanks.


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