Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #67560

Re: Functional programming

Show all headers | View raw

On Monday, March 3, 2014 8:31:47 PM UTC+5:30, Chris Angelico wrote:
> On Tue, Mar 4, 2014 at 1:38 AM, Rustom Mody  wrote:
> > If you want the (semantic) equivalent of python's [1,2,'foo']
> > you need to make an explicit union Int and String and its that
> > *single* union type's elements that must go in.
> > In all cases its always a single type. And so
> > sum([1,2,[3])

> Okay. That's how the declaration goes, then. So how do you tell it
> that 1 isn't an Int, it's a member of the union of Int and String? How
> do you create a list which has [Int_String(1), Int_String(2)] and is
> therefore allowed to be added to [Int_String('foo')] ? Can you do that
> with literals?

Mmmm This is getting a bit OT for a python list
Anyway here goes
The Union type is called Either.
Its got two constructors -- Left and Right -- better to think of them as
tags to not confuse with OO constructors.
The names Left and Right seem to be a bit meaningless because the Either
type is completely generic -- any types S and T can be 'unioned' as
Either S T
where the S components look like Left x for x : S
and the T components look like Right x for x : T

So python's [1,2,"foo"] is
written as
[Left 1, Left 2, Right "foo"]

> This is why it's tricky to put rules in based on type inference. The
> programmer's intent isn't in the picture. If Python ever acquires that
> kind of restriction ("here's a list that can contain only this type /
> these types of object"), I would hope that it's left up to the
> programmer, not the compiler, to stipulate. That's how it is with Pike
> (if you just say "array", it can take anything), and that's the only
> way to be sure the programmer doesn't have to fight the language.

> You said earlier

> >> On Tue, Mar 4, 2014 at 1:08 AM, Rustom Mody wrote:
> >> > If 'integer-less-than-3' were a type then yes there would be this
> >> > problem. More generally, if types could overlap then automatic
> >> > type-inference is impossible

> The type "Int" overlaps with the type "Union of Int and String". How
> is that resolved? 

By decreeing "no overlap!" :-)

Left 1 : Either Int String
whereas 1 : Int

Strictly speaking 'union' should be called 'disjoint union' but this is
so universal in programming that its dropped as redundant.

Heck even C's union is disjoint! If we have

union u
{ 
  int i;
  char c;
};

union u x;

Now you cant interchange the usages of x x.i and x.c

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: Functional programming Ned Batchelder <ned@nedbatchelder.com> - 2014-03-02 20:27 -0500
  Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 03:45 -0800
    Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-03 23:20 +1100
      Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 05:48 -0800
        Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 05:51 -0800
        Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 01:00 +1100
          Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 06:08 -0800
            Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 01:23 +1100
              Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 06:38 -0800
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 02:01 +1100
                Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 07:28 -0800
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-03 17:27 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 05:37 +1100
                Re: Functional programming Steven D'Aprano <steve@pearwood.info> - 2014-03-04 05:35 +0000
                Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 21:59 -0800
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 17:04 +1100
                Re: Functional programming Rustom Mody <rustompmody@gmail.com> - 2014-03-03 22:20 -0800
                Re: Functional programming Steven D'Aprano <steve@pearwood.info> - 2014-03-04 08:56 +0000
                Re: Functional programming Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-04 11:56 +0100
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 11:47 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 00:01 +1100
                OT Sine Rule [was Re: Functional programming] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 14:25 +0000
                Re: OT Sine Rule [was Re: Functional programming] Tim Chase <python.list@tim.thechases.com> - 2014-03-04 08:37 -0600
                Re: OT Sine Rule [was Re: Functional programming] Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-04 14:42 +0000
                Re: OT Sine Rule [was Re: Functional programming] Chris Angelico <rosuav@gmail.com> - 2014-03-05 02:06 +1100
                Re: OT Sine Rule [was Re: Functional programming] Tim Chase <python.list@tim.thechases.com> - 2014-03-04 09:21 -0600
                Re: Functional programming Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-05 09:59 +0100
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 21:49 +0200
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 07:01 +1100
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 22:50 +0200
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 08:06 +1100
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 23:21 +0200
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 08:26 +1100
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 23:43 +0200
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 08:52 +1100
                Re: Functional programming Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-05 12:57 +1300
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 02:11 +0200
                Re: Functional programming "BartC" <bc@freeuk.com> - 2014-03-04 13:30 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 00:47 +1100
                Re: Functional programming Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-04 14:05 +0000
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 14:55 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 02:13 +1100
                Re: Functional programming MRAB <python@mrabarnett.plus.com> - 2014-03-04 17:07 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 01:17 +1100
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 15:18 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 02:28 +1100
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 15:45 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 03:04 +1100
                Re: Functional programming Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-05 10:09 +0100
                Re: Functional programming "BartC" <bc@freeuk.com> - 2014-03-05 11:28 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-05 23:04 +1100
                Re: Functional programming Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 14:11 +0200
                Re: Functional programming Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-03-04 11:06 +1300
                Re: Functional programming Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 09:31 +1100
                Re: Functional programming Grant Edwards <invalid@invalid.invalid> - 2014-03-04 14:59 +0000
                Re: Functional programming Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 15:22 +0000
                Re: Functional programming Chris Angelico <rosuav@gmail.com> - 2014-03-04 09:42 +1100
                Re: Functional programming Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 10:52 +1100
                Re: Functional programming "BartC" <bc@freeuk.com> - 2014-03-04 09:41 +0000
            Re: Functional programming 88888 Dihedral <dihedral88888@gmail.com> - 2014-03-03 16:35 -0800

csiph-web