Comparison operators for strings /and/ numbers?

Comparison operators for strings /and/ numbers?

[Christopher Howard]

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Hi, in another lisp I have been working with, it has <, >, and ==
(structure equality) operators which can take string arguments, number
arguments, or a mixture of both. But it seems in guile that there are
separate comparison operators for strings and for numbers. This makes
sense but is not very convenient for my present purpose. Is there some
other guile operators or extension operators that will handle both? I
could make some I'm sure, but I don't want to reinvent the wheel.

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Re: Comparison operators for strings /and/ numbers?

[David Kastrup]

Christopher Howard <christopher.howard@qlfiles.net> writes:

> Hi, in another lisp I have been working with, it has <, >, and ==
> (structure equality) operators which can take string arguments, number
> arguments, or a mixture of both. But it seems in guile that there are
> separate comparison operators for strings and for numbers. This makes
> sense but is not very convenient for my present purpose. Is there some
> other guile operators or extension operators that will handle both? I
> could make some I'm sure, but I don't want to reinvent the wheel.

(use-modules (oop goops))

(define-method (< (a <string>) . rest)
  (apply string<? a rest))

(< "g" "b") ;; => #f

-- 
David Kastrup

Re: Comparison operators for strings /and/ numbers?

[Christopher Howard]

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On Thu, 2017-08-24 at 10:05 +0200, David Kastrup wrote:
> Christopher Howard <christopher.howard@qlfiles.net> writes:
> 
> > Hi, in another lisp I have been working with, it has <, >, and ==
> > (structure equality) operators which can take string arguments,
> > number
> > arguments, or a mixture of both. But it seems in guile that there
> > are
> > separate comparison operators for strings and for numbers. This
> > makes
> > sense but is not very convenient for my present purpose. Is there
> > some
> > other guile operators or extension operators that will handle both?
> > I
> > could make some I'm sure, but I don't want to reinvent the wheel.
> 
> (use-modules (oop goops))
> 
> (define-method (< (a <string>) . rest)
>   (apply string<? a rest))
> 
> (< "g" "b") ;; => #f
> 

Forgive me for bring this thread back, but I just finished reading the
goops info manual...

My question, specifically: Suppose:

1) I use fn "<" in my module, as a goops generic, and then
2) somebody else "use"s my module in their module, and then
3) suppose they do a "define-method" to create another "<" for a new
datatype
4) they call a function in my module that uses "<"

Will that function call have access to the new method for that generic?

Background: This would obvious be very important in a module providing
a generic data structure (like a binary tree), where you would want to
compare keys using "<", but you wouldn't want to arbitrarily limit the
possible data types to what was defined in the module.

Or do I need to define an <ordered> class and expose that to my module
users? I'm thinking in terms like Data.Ord class from Haskell. (Java
folks would call it an "interface".)

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Re: Comparison operators for strings /and/ numbers?

[Arne Babenhauserheide]

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Hi Christopher,

You can use GOOPS to make them generic:

> (import (oop goops))
> (< "a" "b")
<unnamed port>:3:0: <unnamed port>:3:0: In procedure <: Wrong type argument in position 1: "a"

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
[1]> (define-method (< (a <string>) (b <string>)) (string<? a b))
[1]> (< "a" "b")
$1 = #t

For some functions you might need to call (define-generic <function>)
before this (you’ll notice because define-method will signal an error).

Best wishes,
Arne

Christopher Howard <christopher.howard@qlfiles.net> writes:

> Hi, in another lisp I have been working with, it has <, >, and ==
> (structure equality) operators which can take string arguments, number
> arguments, or a mixture of both. But it seems in guile that there are
> separate comparison operators for strings and for numbers. This makes
> sense but is not very convenient for my present purpose. Is there some
> other guile operators or extension operators that will handle both? I
> could make some I'm sure, but I don't want to reinvent the wheel.

-- 
Unpolitisch sein
heißt politisch sein
ohne es zu merken

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Re: Comparison operators for strings /and/ numbers?

[Ralf Mattes]

On Thu, Aug 24, 2017 at 10:29:41AM +0200, Arne Babenhauserheide wrote:
> Hi Christopher,
> 
> You can use GOOPS to make them generic:
> 
> > (import (oop goops))
> > (< "a" "b")

That's not what David suggested. Of course you still need to define the
method specialized on strings.

> <unnamed port>:3:0: <unnamed port>:3:0: In procedure <: Wrong type argument in position 1: "a"
> 
> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> [1]> (define-method (< (a <string>) (b <string>)) (string<? a b))
> [1]> (< "a" "b")
> $1 = #t
> 
> For some functions you might need to call (define-generic <function>)
> before this (you’ll notice because define-method will signal an error).

Is that still the case? IIRC this was true for Guile 1 where you needed
to "redeclare" build-in functions as generic, but from what I see Guile2
automatically generates the appropriate generic functions. This is
pretty much how Common Lisp handles defmethod without a matching
defgeneric (but in CL you aren't allowed to transform a function from CL 
into a generic).

Cheers, Ralf Mattes

> Best wishes,
> Arne
> 
> Christopher Howard <christopher.howard@qlfiles.net> writes:
> 
> > Hi, in another lisp I have been working with, it has <, >, and ==
> > (structure equality) operators which can take string arguments, number
> > arguments, or a mixture of both. But it seems in guile that there are
> > separate comparison operators for strings and for numbers. This makes
> > sense but is not very convenient for my present purpose. Is there some
> > other guile operators or extension operators that will handle both? I
> > could make some I'm sure, but I don't want to reinvent the wheel.
> 
> -- 
> Unpolitisch sein
> heißt politisch sein
> ohne es zu merken

Re: Comparison operators for strings /and/ numbers?

[Arne Babenhauserheide]

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Ralf Mattes <rm@seid-online.de> writes:

> On Thu, Aug 24, 2017 at 10:29:41AM +0200, Arne Babenhauserheide wrote:
>> Hi Christopher,
>> 
>> You can use GOOPS to make them generic:
>> 
>> > (import (oop goops))
>> > (< "a" "b")
>
> That's not what David suggested.

No, it is not. I wrote my reply before I saw his.

>> <unnamed port>:3:0: <unnamed port>:3:0: In procedure <: Wrong type argument in position 1: "a"
>> 
>> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
>> [1]> (define-method (< (a <string>) (b <string>)) (string<? a b))
>> [1]> (< "a" "b")
>> $1 = #t
>> 
>> For some functions you might need to call (define-generic <function>)
>> before this (you’ll notice because define-method will signal an error).
>
> Is that still the case?

Yes. Try:

> (import (oop goops))
> (define-method (list (a <string>)))
ERROR: In procedure scm-error:
ERROR: #<procedure list _> is not a valid generic function

Best wishes,
Arne
-- 
Unpolitisch sein
heißt politisch sein
ohne es zu merken

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Re: Comparison operators for strings /and/ numbers?

[Chris Vine]

On Wed, 23 Aug 2017 22:05:37 -0800
Christopher Howard <christopher.howard@qlfiles.net> wrote:
> Hi, in another lisp I have been working with, it has <, >, and ==
> (structure equality) operators which can take string arguments, number
> arguments, or a mixture of both. But it seems in guile that there are
> separate comparison operators for strings and for numbers. This makes
> sense but is not very convenient for my present purpose. Is there some
> other guile operators or extension operators that will handle both? I
> could make some I'm sure, but I don't want to reinvent the wheel.

The predicate equal? will compare numbers, strings and records.

https://www.gnu.org/software/guile/docs/master/guile.html/Equality.html#index-equal_003f

Chris

Re: Comparison operators for strings /and/ numbers?

[Chris Vine]

On Thu, 24 Aug 2017 14:31:41 +0100
Chris Vine <vine35792468@gmail.com> wrote:
> On Wed, 23 Aug 2017 22:05:37 -0800
> Christopher Howard <christopher.howard@qlfiles.net> wrote:
> > Hi, in another lisp I have been working with, it has <, >, and ==
> > (structure equality) operators which can take string arguments,
> > number arguments, or a mixture of both. But it seems in guile that
> > there are separate comparison operators for strings and for
> > numbers. This makes sense but is not very convenient for my present
> > purpose. Is there some other guile operators or extension operators
> > that will handle both? I could make some I'm sure, but I don't want
> > to reinvent the wheel.  
> 
> The predicate equal? will compare numbers, strings and records.
> 
> https://www.gnu.org/software/guile/docs/master/guile.html/Equality.html#index-equal_003f

I meant to add, but failed to do so, that as far as I am aware there is
no built-in polymorphic < or > procedure that will take strings as well
as numbers.  As a simple work-around you could use type predicates in a
cond expression to dispatch the appropriate comparison procedure for
the type in question.

Chris