[Haskell-cafe] Re: Fwd: Is () a 0-length tuple?
jon.fairbairn at cl.cam.ac.uk
Sun Nov 8 05:14:07 EST 2009
"Pasqualino \"Titto\" Assini" <tittoassini at gmail.com> writes:
> The syntax is similar, but what else is?
What would you expect from an empty tuple?
(a,b,c) has a constructor function p3 a b c = (a,b,c) and three
destructor functions s3_1 (a,b,c) = a, s3_2 (a,b,c) = b and s3_3 (a,b,c)=c
(a,b) has a constructor function p2 a b = (a,b) and two destructor functions
s2_1 (a,b) = a and s2_2 (a,b) = b
(a) has a constructor function p1 a = (a) and one destructor function
s1_1 a = a
() has a constructor function p0 = () and zero destructor functions.
> null type.
Haskell (null type would seem more like empty than single), but anyway,
the thing is that the sole (non-bottom) value of the /unit/ type is the
same as the empty tuple¹.
> Isn't () the same thing? The only value of the unary type?
The "empty" type in Haskell would be (forall a.a) which has no
 Aside: I wanted haskell to share the same type for all empty values
(ie lists would have been symmetric unions of pairs with the unit type
List t = (t,List t) || ()), but that didn't fit with algebraic datatypes
so the design went a different way.
Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk
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