[Ur] int to string?

Sun Oct 9 07:38:17 EDT 2016

Hi Michael,

I believe that happens because the module List binds a function to the show
variable:

val show = fn [a] (_ : show a) =>
              let
                  fun show' (ls : list a) =
                      case ls of
                          [] => "[]"
                        | x :: ls => show x ^ " :: " ^ show' ls
              in
                  mkShow show'
              end

Therefore, opening the List module shadows the show function defined in the
Basis module.

val show : t ::: Type -> show t -> t -> string

Regards,
Saulo

On Sun, Oct 9, 2016 at 6:55 AM, Michael Rohs <
michael.rohs at hci.uni-hannover.de> wrote:

> Hi Saulo,
>
> Thanks for your help. Thanks for the suggestion to add type annotations.
> It indeed works, but only if I don't open the List module.
>
> The problem appears if I add module List (even without using any of its
> functions).
>
> test.urp:
> $/list
> test
>
> test.ur:
> open List
>
> if I remove "open List" from test.ur it works.
>
> I don't understand why...
>
> Here is the full test.ur:
> ----------------------------------------------
> (*open List*)  <-- error if uncommenting this line
>
> fun int2string (i : int) : string = show i
> val s = int2string 123
>
> fun main () =
>     let
>         val stuff = "apple" :: "key" :: "goat" :: []
>         fun predicate (s : string) : bool = s <> "apple"
>         (* val stuff2 = List.filter predicate stuff*)
>     in
>         return <xml><body>
>         {[s]}<br/>
>           <br/>
>         </body></xml>
>     end
> ----------------------------------------------
>
> Best,
> Michael
>
>
> > On 09 Oct 2016, at 10:04, Saulo Araujo <saulo2 at gmail.com> wrote:
> >
> > Hi Michael,
> >
> > Your definition of int2string is fine. For example, the code
> >
> > fun int2string (i : int) : string = show i
> >
> > val s = int2string 10
> >
> > compiles without errors. I suspect the problem is in another part of
> your code. The error message suggests that you have an expression that
> produces a list where something else is expected. In my experience learning
> languages of the ML family, in situations like this, it helps to
> type-annotate arguments and results of functions. Eventually, it also helps
> to type-annotate expressions. For example
> >
> > val h = 10
> > val t = []
> > val s = (h :: t) : int
> >
> > Thanks to the type annotation ": int", the compiler will produce an
> error saying that there is a list int where an int is required:
> >
> > Expression:  Basis.Cons [int] {1 = h, 2 = t}
> >   Have con:  list int
> >   Need con:  int
> > Incompatible constructors
> > Have:  list int
> > Need:  int
> >
> > Regards,
> > Saulo
> >
> > On Sun, Oct 9, 2016 at 4:07 AM, Michael Rohs <michael.rohs at hci.uni-
> hannover.de> wrote:
> > Hi all,
> >
> > I have a question to a very simple problem. How to convert an integer to
> a string?
> >
> > This does not work:
> >
> > fun int2string (i : int) : string = show i
> >
> > Error message:
> >
> > /.../test.ur:19:36: (to 19:40) Unification failure
> > Expression:  show [<UNIF:H::Type>] _
> >   Have con:  show (list <UNIF:H::Type>)
> >   Need con:  <UNIF:I::Type> -> <UNIF:J::Type>
> > Incompatible constructors
> > Have:  show (list <UNIF:H::Type>)
> > Need:  <UNIF:I::Type> -> <UNIF:J::Type>
> >
> > I couldn't find anything else like Int.toString or so.
> >
> > Thank you very much in advance!
> >
> > Best,
> > Michael
> >
> >
> > _______________________________________________
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> >
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