[Caml-list] format strings

[Caml-list] format strings

[Jiten Pathy]

When i define a wrapper around format functions like

let f s = (fun x -> Printf.sprintf s x);;

val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>

I am confused about the inferred type. where does this 'b comes from?

Re: [Caml-list] format strings

[John F Carr]

Your wrapper returns a function.  Type ‘a, the argument to that function, is determined by the first %-conversion in the format string.  The function returns type ‘b.  If your format argument has one %-conversion, type ‘b will be a string.  If it has more than one, type ‘b will be a function accepting the remaining arguments.

# let g = f “%d %d”;;
val g : int -> int -> string = <fun>
# let h = g 0;;
val h : int -> string = <fun>
# h 1;;
- : string = "0 1"

In the past partial application of printf family functions hasn’t worked well, so test before relying on behavior of the returned function.

> On Feb 9, 2015, at 06:58 , Jiten Pathy <jpathy@fssrv.net> wrote:
> 
> When i define a wrapper around format functions like
> 
> let f s = (fun x -> Printf.sprintf s x);;
> 
> val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
> 
> I am confused about the inferred type. where does this 'b comes from?
> 
> -- 
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs

Re: [Caml-list] format strings

[Jiten Pathy]

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Right. what confuses me is, how the format type has 'a -> 'b instead of 'a.
On Feb 9, 2015 4:25 AM, "John F Carr" <jfc@mit.edu> wrote:

> Your wrapper returns a function.  Type ‘a, the argument to that function,
> is determined by the first %-conversion in the format string.  The function
> returns type ‘b.  If your format argument has one %-conversion, type ‘b
> will be a string.  If it has more than one, type ‘b will be a function
> accepting the remaining arguments.
>
> # let g = f “%d %d”;;
> val g : int -> int -> string = <fun>
> # let h = g 0;;
> val h : int -> string = <fun>
> # h 1;;
> - : string = "0 1"
>
> In the past partial application of printf family functions hasn’t worked
> well, so test before relying on behavior of the returned function.
>
> > On Feb 9, 2015, at 06:58 , Jiten Pathy <jpathy@fssrv.net> wrote:
> >
> > When i define a wrapper around format functions like
> >
> > let f s = (fun x -> Printf.sprintf s x);;
> >
> > val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
> >
> > I am confused about the inferred type. where does this 'b comes from?
> >
> > --
> > Caml-list mailing list.  Subscription management and archives:
> > https://sympa.inria.fr/sympa/arc/caml-list
> > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> > Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>

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RE: [Caml-list] format strings

[David Allsopp]

Jiten Pathy wrote:
> When i define a wrapper around format functions like
> 
> let f s = (fun x -> Printf.sprintf s x);;
> 
> val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
> 
> I am confused about the inferred type. where does this 'b comes from?

Your wrapper carries the requirement that the format [s] must have at least one %-parameter. Writing without the fun might make this more obvious:

let f s x = Printf.sprintf s x

The 'a -> 'b is the encoding of this requirement. f "foo" will type. If instead you wrote:

let f s = Printf.sprintf s

you'd get the type you're expecting: ('a, unit, bytes) format -> 'a

and you can have format strings with no %-parameters (so f "foo" now types). If you meant to have it so that the format string must have *exactly* one %-parameter then you need an annotation:

let f s x : string = Printf.sprintf s x

and now f "%d" will type, but f "%s %d" will not.

HTH,


David

Re: [Caml-list] format strings

[Jiten Pathy]

Is there a way I can define a function which has the same signature of
printf family but is a no-op?

f : ('a, 'b, unit) format -> 'a

assert(f "%s %d" "hello" 1 = ());;

On Mon, Feb 9, 2015 at 9:32 AM, David Allsopp <dra-news@metastack.com> wrote:
> Jiten Pathy wrote:
>> When i define a wrapper around format functions like
>>
>> let f s = (fun x -> Printf.sprintf s x);;
>>
>> val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
>>
>> I am confused about the inferred type. where does this 'b comes from?
>
> Your wrapper carries the requirement that the format [s] must have at least one %-parameter. Writing without the fun might make this more obvious:
>
> let f s x = Printf.sprintf s x
>
> The 'a -> 'b is the encoding of this requirement. f "foo" will type. If instead you wrote:
>
> let f s = Printf.sprintf s
>
> you'd get the type you're expecting: ('a, unit, bytes) format -> 'a
>
> and you can have format strings with no %-parameters (so f "foo" now types). If you meant to have it so that the format string must have *exactly* one %-parameter then you need an annotation:
>
> let f s x : string = Printf.sprintf s x
>
> and now f "%d" will type, but f "%s %d" will not.
>
> HTH,
>
>
> David

Re: [Caml-list] format strings

[Jiten Pathy]

nvm. make_printf is what I am after. Some complicated stuff is going
on in CamlInternalFormat.

On Mon, Feb 9, 2015 at 5:05 PM, Jiten Pathy <jpathy@fssrv.net> wrote:
> Is there a way I can define a function which has the same signature of
> printf family but is a no-op?
>
> f : ('a, 'b, unit) format -> 'a
>
> assert(f "%s %d" "hello" 1 = ());;
>
> On Mon, Feb 9, 2015 at 9:32 AM, David Allsopp <dra-news@metastack.com> wrote:
>> Jiten Pathy wrote:
>>> When i define a wrapper around format functions like
>>>
>>> let f s = (fun x -> Printf.sprintf s x);;
>>>
>>> val f : ('a -> 'b, unit, bytes) format -> 'a -> 'b = <fun>
>>>
>>> I am confused about the inferred type. where does this 'b comes from?
>>
>> Your wrapper carries the requirement that the format [s] must have at least one %-parameter. Writing without the fun might make this more obvious:
>>
>> let f s x = Printf.sprintf s x
>>
>> The 'a -> 'b is the encoding of this requirement. f "foo" will type. If instead you wrote:
>>
>> let f s = Printf.sprintf s
>>
>> you'd get the type you're expecting: ('a, unit, bytes) format -> 'a
>>
>> and you can have format strings with no %-parameters (so f "foo" now types). If you meant to have it so that the format string must have *exactly* one %-parameter then you need an annotation:
>>
>> let f s x : string = Printf.sprintf s x
>>
>> and now f "%d" will type, but f "%s %d" will not.
>>
>> HTH,
>>
>>
>> David

RE: [Caml-list] format strings

[David Allsopp]

Jiten Pathy wrote:
> nvm. make_printf is what I am after.

No it isn't - using internal functions is, to 1 significant figure, never what you're after. Apart from anything, you're instantly forcing your code to be 4.02+

> Some complicated stuff is going on in CamlInternalFormat.

Indeed - 4.02 has a very clever reimplementation of the printf engine.

> On Mon, Feb 9, 2015 at 5:05 PM wrote:
> > Is there a way I can define a function which has the same signature of
> > printf family but is a no-op?
> >
> > f : ('a, 'b, unit) format -> 'a

Printf.ifprintf (you can do similar with Printf.ksprintf and Printf.ikfprintf).

I don't know what lead you to read CamlInternalFormat.mli, but the reference manual (http://caml.inria.fr/pub/docs/manual-ocaml-4.02/libref/Printf.html) is a much better place to start for information on these modules than their source code. 


David

Re: [Caml-list] format strings

[Jiten Pathy]

Ok, If i am not supposed to use make_printf, then what is the
recommended way of writing the following?

This adds a new line to given format string. In particular how do i
manipulate the format strings passed to me?


open CamlinternalFormat;;
let f (Format (fmt, _)) =
  make_printf (fun o acc -> output_acc o (Acc_char_literal (acc,
'\n')); ignore o) stdout End_of_acc fmt
;;
val f : ('a, out_channel, unit, unit, unit, unit) format6 -> 'a = <fun>

utop # f "%s %d" "# " 3;;
#  3
- : unit = ()

I now have a slightly better understanding of CamlinternalFormat.



On Mon, Feb 9, 2015 at 11:44 PM, David Allsopp <dra-news@metastack.com> wrote:
> Jiten Pathy wrote:
>> nvm. make_printf is what I am after.
>
> No it isn't - using internal functions is, to 1 significant figure, never what you're after. Apart from anything, you're instantly forcing your code to be 4.02+
>
>> Some complicated stuff is going on in CamlInternalFormat.
>
> Indeed - 4.02 has a very clever reimplementation of the printf engine.
>
>> On Mon, Feb 9, 2015 at 5:05 PM wrote:
>> > Is there a way I can define a function which has the same signature of
>> > printf family but is a no-op?
>> >
>> > f : ('a, 'b, unit) format -> 'a
>
> Printf.ifprintf (you can do similar with Printf.ksprintf and Printf.ikfprintf).
>
> I don't know what lead you to read CamlInternalFormat.mli, but the reference manual (http://caml.inria.fr/pub/docs/manual-ocaml-4.02/libref/Printf.html) is a much better place to start for information on these modules than their source code.
>
>
> David

RE: [Caml-list] format strings

[David Allsopp]

Jiten Pathy wrote:
> Ok, If i am not supposed to use make_printf, then what is the recommended
> way of writing the following?
> 
> This adds a new line to given format string. In particular how do i
> manipulate the format strings passed to me?

There wasn't a vast amount of type-safe manipulation possible with format strings in the old implementation, though given the newer representation, perhaps something may come in the future. 

> open CamlinternalFormat;;
> let f (Format (fmt, _)) =
>   make_printf (fun o acc -> output_acc o (Acc_char_literal (acc, '\n'));
> ignore o) stdout End_of_acc fmt ;; val f : ('a, out_channel, unit, unit,
> unit, unit) format6 -> 'a = <fun>
> 
> utop # f "%s %d" "# " 3;;
> #  3
> - : unit = ()

However, for this "simple" manipulation there are two ways which don't touch the internals:

let f fmt = Printf.ksprintf (Printf.printf "%s\n") fmt

[that's my preference - to me it's clearer that you're wrapping the formatted output. I'd often do this to add %! as well - i.e. "%s\n%!"]

let f fmt = Printf.printf (fmt ^^ "\n")

[See http://caml.inria.fr/pub/docs/manual-ocaml-4.02/libref/Pervasives.html#6_Operationsonformatstrings]


David

Re: [Caml-list] format strings

[Jiten Pathy]

Thanks, that helps. I can't think of any complicated manipulation i
would need, for now.

On Tue, Feb 10, 2015 at 12:25 AM, David Allsopp <dra-news@metastack.com> wrote:
> Jiten Pathy wrote:
>> Ok, If i am not supposed to use make_printf, then what is the recommended
>> way of writing the following?
>>
>> This adds a new line to given format string. In particular how do i
>> manipulate the format strings passed to me?
>
> There wasn't a vast amount of type-safe manipulation possible with format strings in the old implementation, though given the newer representation, perhaps something may come in the future.
>
>> open CamlinternalFormat;;
>> let f (Format (fmt, _)) =
>>   make_printf (fun o acc -> output_acc o (Acc_char_literal (acc, '\n'));
>> ignore o) stdout End_of_acc fmt ;; val f : ('a, out_channel, unit, unit,
>> unit, unit) format6 -> 'a = <fun>
>>
>> utop # f "%s %d" "# " 3;;
>> #  3
>> - : unit = ()
>
> However, for this "simple" manipulation there are two ways which don't touch the internals:
>
> let f fmt = Printf.ksprintf (Printf.printf "%s\n") fmt
>
> [that's my preference - to me it's clearer that you're wrapping the formatted output. I'd often do this to add %! as well - i.e. "%s\n%!"]
>
> let f fmt = Printf.printf (fmt ^^ "\n")
>
> [See http://caml.inria.fr/pub/docs/manual-ocaml-4.02/libref/Pervasives.html#6_Operationsonformatstrings]
>
>
> David