[Haskell-beginners] questions about product types

AntC anthony_clayden at clear.net.nz
Wed Jan 18 23:09:40 CET 2012

Tom Doris <tomdoris <at> gmail.com> writes:

> 3) In type theory, are product types consisting of distinct member types 
considered special in any way? i.e. does the concept have a name and what 
special properties do they have?
I don't know about type theory, but I've seen them called 'Type Indexed 
Products'. See the HList Paper [1] section 7, which refers to [31].

I've found TIP's very successful in practice. They've gotten more rugged 
through the gradual innovations in type inference that have been introduced 
into GHC. (Especially the type equality constraint (~), which improves on 
HList's TypeCast.) But the 'groundness issues' in HList section 9 still apply.

Using TIP's relies on overlapping instances (but _not_ necessarily Functional 
Dependencies), so it's still a 'poor cousin' and (it seems) not to be 
mentioned in polite company.

[1] Strongly Typed Heterogenous Collections, August 2004, 
[31] Type-indexed Rows, POPL 2001, Shields/Meijer

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