Re: [Caml-list] Printf.sprintf ?

Re: [Caml-list] Printf.sprintf ?

[Johann Spies]

Dave Mason <dmason@sarg.ryerson.ca> writes:

> But unless you force it to unify as a format, it is treated as a
> string, so you get the effect you saw.

Thanks Dave.  And how do I do that (force it to unify as a format)?

I want to use this to build a sql-query recursively from a list.


Johann
-- 
Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "Praying always with all prayer and supplication in the
      Spirit, and watching thereunto with all perseverance 
      and supplication for all saints."       Ephesians 6:18
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[Caml-list] Printf.sprintf ?

[Johann Spies]

Can somebody please explain this to me:

This works as expected:

# let s = Printf.sprintf "%s" "enigiets";;
val s : string = "enigiets"

But this not:

# let q = "%s";;
val q : string = "%s"
# let s = Printf.sprintf q "enigiets";;
Characters 23-24:
This expression has type string but is here used with type
  (string -> 'a, unit, string) format

Why?

Johann
-- 
Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "Praying always with all prayer and supplication in the
      Spirit, and watching thereunto with all perseverance 
      and supplication for all saints."       Ephesians 6:18
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Re: [Caml-list] Printf.sprintf ?

[Brian Rogoff]

On 15 Aug 2001, Johann Spies wrote:

> Can somebody please explain this to me:
> 
> This works as expected:
> 
> # let s = Printf.sprintf "%s" "enigiets";;
> val s : string = "enigiets"
> 
> But this not:
> 
> # let q = "%s";;
> val q : string = "%s"
> # let s = Printf.sprintf q "enigiets";;
> Characters 23-24:
> This expression has type string but is here used with type
>   (string -> 'a, unit, string) format
> 
> Why?

Because Printf is a bit of a hack, in that the syntax for a format string
is the same as the syntax for a regular string. As a beginner, when you
use Printf, it's easier to use the format string as a literal and not
as a variable. If you feel you must do this, the polite way is as follows 

# let q = ("%s" : ('a, 'b, 'c) format);;  
val q : (string -> 'a, 'b, 'a) format = <abstr>
# let s = Printf.sprintf q "enigiets";;
val s : string = "enigiets"

and there's also a rude way (forgive me Berke! :) 

  let s : string = Printf.sprintf (Obj.magic q) "enigiets";;

-- Brian

(*) Yes, I'm a C programmer too so it's a welcome hack. 

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Re: [Caml-list] Printf.sprintf ?

[Remi VANICAT]

Brian Rogoff <bpr@best.com> writes:

> Because Printf is a bit of a hack, in that the syntax for a format string
> is the same as the syntax for a regular string. As a beginner, when you
> use Printf, it's easier to use the format string as a literal and not
> as a variable. If you feel you must do this, the polite way is as follows 
> 
> # let q = ("%s" : ('a, 'b, 'c) format);;  
> val q : (string -> 'a, 'b, 'a) format = <abstr>
> # let s = Printf.sprintf q "enigiets";;
> val s : string = "enigiets"
> 
> and there's also a rude way (forgive me Berke! :) 
> 
>   let s : string = Printf.sprintf (Obj.magic q) "enigiets";;

it's often not a good idea : 

# let q = "%s";;
val q : string = "%s"
# let s : string = Printf.sprintf (Obj.magic q)0;;        
Segmentation fault
moi@debian:~$

this break the type system.

better use the first solution.
-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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