On Fri, 2008-12-26 at 11:51 +0000, Thomas DuBuisson wrote:
Hello cafe, This is just a small thought, but its been bugging me. We have these things called type classes for a reason (I like to think).
Type classes were invented for two reasons: 1) To imitate mathematical convention. Addition, in full, is written as x +_{A} y where A is a mathematical structure supplying addition. However, the convention is that the subscript may be omitted `when no ambiguity may arise'. Programming languages (generally) take this and run with it, allowing x + y to mean (depending on the language) pretty much anything. Type classes are an ingenious step back toward the mathematical convention, where the operation + must come from some complete structure. 2) To allow conversion from structured data into strings to be treated as a single operation. Most languages support this in some form, but I am increasingly failing to see why. There are usually several different ways in which a given piece of structured data can meaningfully be `shown'; in languages which try to do the right thing when given print "string", 'c', (2 + 2), [true, false, :maybe] you *still* end up defining (multiple!) special-purpose output or conversion-to-string functions so you can print the same data in multiple ways. I think making Show a type class was a mistake. jcc