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Subject: Re: Subtype conformance... not what I was expecting.
From: Georg Bauhaus <bauhaus@futureapps.de>
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Date: Wed, 26 Jul 2006 17:38:50 +0200
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Date: 2006-07-26T17:35:54+02:00
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On Wed, 2006-07-26 at 10:32 -0400, Peter C. Chapin wrote:

Sounds like the technical/mathematical versus practical/real-world
co(ntra)variance issue?

> Using the notation common in functional languages, let F : t1 -> t2 and 
> G : t1' -> t2'. Then t1 -> t2 is a subtype of t1' -> t2' (that is, 
> t1->t2 <: t1'->t2') iff t2 <: t2' and t1' <: t1. In this case I realize 
> that G is an access type but it "feels" like a function when it is used. 
> Since I'm trying to use F where G is expected and since F and G have the 
> proper subtype relationship to each other one might suppose that the 
> above rule would be obeyed.

Let F : Person -> Room and G : Girl -> Girls_Dormitory.
Then Person -> Room is a subtype of Girl -> Girls_Dormitory (that is,
Person->Room <: Girl->Girls_Dormitory) iff Room <: Girls_Dormitory
and Girl <: Person.

Let F : Girl -> Girls_Dormitory and G : Person -> Room.
Then Girl -> Girls_Dormitory is a subtype of Person -> Room (that is,
Girl->Girls_Dormitory <: Person->Room) iff Girls_Dormitory <: Room
and Person <: Girl.

Now add Boy.
Granted Persons and Rooms are distinct type classes, but are the
transcriptions correct otherwise? (The example is from Meyer's OOSC2
as I remember it, but I don't have the book here to check.)