# SJT's book has a limited definition of relation... why?

Submitted by metaperl on Tue, 10/04/2005 - 1:58pm.

In 16.9 of SJT's book "Craft of Functional Programming" he says "A binary relation relates together certain elements of a set"

My problem with this is that the elements do not have to be in the same set. A relation is a subset of the cross product of **sets** A and B, where A and B are not of the same type.

Next he gives a type definition

`type Relation a = Set (a,a)`

And I think it should be:

`type Relation a,b = Set (a,b)`

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I went to irc://irc.efnet.org/math and asked them and they agreed with me:

[12:43:30] I would like a definition for a relation. I want to also verify that the range of a relation need not be the same data type as the domain

[12:44:24] the range does not need to be the same type as domain

[12:44:53] <@monochrom> Let X, Y be sets or data types. A relation is a function taking two arguments, one of type X and the other of type Y, and the result is a boolean.

[12:45:08] I didnt think so, but in a functional programming book I am studying he says it is: type Relation a = Set (a,a) --- in my eyes it could be type Relation a,b = Set (a,b)

[12:45:41] where a and b are types, perhaps different

[12:46:20] A relation is a subset of the cross-product A x B, where A and B are arbitrary sets (they do not need to be of the same data type)

[12:46:29] <@monochrom> Example. There is a group of people X and a collection of icecream flavours Y. Do you (a member of X) like strawberry icecream (a member of Y)? Yes or No? True or false?

[12:47:14] beautiful! THANK YOU! I knew he was wrong!

[12:47:22] i guess range and domain are misnomers in the context of *relations*. at any rate i don't see a reason to throw poop at the author if he decides to just look at relations on a single set :p

[12:47:32] <@monochrom> In *my* functional programming book I have type Relation a b = a -> b -> Bool, a formal way to say what I said.

[12:47:58] monochrom: which one? I'm reading Simon J Thompson "Haskell: The Craft of Functional Programming"

[12:48:35] I think I want to write to the author about this

[12:48:50] R = relation between A and B <=> R = { (a, b) | a \in A' ^ b \in B', where A' \subseteq A, B' \subseteq B } <=> R \subseteq A x B

[12:50:13] <@monochrom> I am according to the way things are done in the theorem prover HOL.

[12:51:11] <@monochrom> So I guess the book is the "HOL Description Manual". This HOL is embedded in an ML programming environment, so it is still functional programming.

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