[Haskell-cafe] Type Theory? Relations

Scott Turner p.turner at computer.org
Mon Jul 26 18:21:10 EDT 2004

On 2004 July 26 Monday 13:46, haskell at alias.spaceandtime.org wrote:
> According to Enderton, one of the ways to define an ordered pair (a,b)
> is {{a},{a,b}}.  A relation is defined as a set of ordered-pairs.  A
> map, of course, is a single-valued relation.

The motivation for defining ordered pairs that way is more mathematical than 
type-theoretic.   It arises from having sets as a starting point, and needing 
to define ordered pairs, relations, and functions.

> Given all that, suppose I have a "FiniteMap Int String" in Haskell.
> This is, according to the definitions above, a "Set (Int,String)".   

You have run into a problem expressing your meaning, because (Int, String) 
indicates a specific type in Haskell which is _not_ a Set.  

> An 
> element of that has type (Int,String), which contains {Int,String}.  But
> that can't exist because a Set contains only elements of one type.

The ordered pair 1,"one" would be represented as {{1},{1,"one"}}. Now, 
{1,"one"} can't exist in Haskell as you say, but it can be represented using 
the Either type constructor. 

Either enables a value to be chosen from two otherwise incompatible types. 
Either Int String is a type which can have values that are Ints or Strings, 
but the value must specify which using the Left or Right constructor.
    Left 5 and Right "five" 
are both values of the type Either Int String.
    Left "five"
would be invalid.

Instead of {1,"one"), in Haskell you would have {Left 1, Right "one"} 
of type Set (Either Int String). The ordered pair would be
   {Left {1}, Right {Left 1, Right "one"}}
of type
  Set (Either Int (Either Int String))
and the finite map would be
  Set (Set (Either Int (Either Int String)))

Few people would be able to tolerate writing a program using this type. :-)

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