Using Monads in C++ to Solve Constraints: 1. The List Monad • Hacker News Board

Re: Using Monads in C++ to Solve Constraints: 1. The List Monad

#71

Post by dllthomas » Tue, May 19, 2015, 4:35 PM UTC

mafribe wrote: post #69

tome wrote: post #68

Earlier quoted context omitted.

I don't think anyone knows a solid definition of purity. It certainly refers to at least two things (1. independence of order of evaluation, 2. no side effects) and informally speaking we often "know" what it is in a particular context, but writing down a formal definition is very hard. I'm certainly not satisfied with Sabry's definition. I don't like the idea that you have to have both a strict and a non-strict eval…

If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... Anyway, I'm a bit unhappy about the requirement of full evaluation order independence, because even the purest of pure languages (e.g. the untyped lambda-calculus or PCF) don't have full evaluation order independence. I guess you mean evaluation order independence modulo termination? My intuiti…

Usually this is the way words work - we notice a regularity in the environment we wish to refer to; we build a sense of it, usually motivated by some examples and counterexamples; we give it a handle; and we then try to find a definition that matches our intuition. There certainly seem to be regularities that "pure" is intended to point at in this context, but it definitely seems that we've not settled on a definition, and we may not even agree on examples.

The notions of purity I was pointing at was any that entailed the restrictions in the paper you'd linked. There are restrictions of Haskell that are pure in those senses, but Haskell taken as a whole is not, even without invoking anything unsafe.

Re: Using Monads in C++ to Solve Constraints: 1. The List Monad

#72

Post by dllthomas » Tue, May 19, 2015, 4:55 PM UTC

TheLoneWolfling wrote: post #2

This method is really inefficient. There are b! / (b - n)! ways to choose n of b unique numbers, whereas this solution is O(b^n). That being said, it's still fast enough for this example. Also, Python solution along the same lines: import itertoolsdef def valueOf(*values, base=10): v = 0 for v in itertools.accumulate(values, lambda a,b: a*base+b): pass return v def solve(): for s,e,n,d,m,o,r,y in itertools.permutatio…

Note that the later parts of this series havd been published, and in fact the definition of StateL does inforce uniqueness as expected, in a way that makes the proposed solution O(n!).

For details, look at the first code block in the "The Client Side" section, here: http://bartoszmilewski.com/2015/05/18/using-monads-in-c-to-s...

Re: Using Monads in C++ to Solve Constraints: 1. The List Monad

#73

Post by tome » Tue, May 19, 2015, 10:22 PM UTC

mafribe wrote: post #69

tome wrote: post #68

Earlier quoted context omitted.

I don't think anyone knows a solid definition of purity. It certainly refers to at least two things (1. independence of order of evaluation, 2. no side effects) and informally speaking we often "know" what it is in a particular context, but writing down a formal definition is very hard. I'm certainly not satisfied with Sabry's definition. I don't like the idea that you have to have both a strict and a non-strict eval…

If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ... Anyway, I'm a bit unhappy about the requirement of full evaluation order independence, because even the purest of pure languages (e.g. the untyped lambda-calculus or PCF) don't have full evaluation order independence. I guess you mean evaluation order independence modulo termination? My intuiti…

> If nobody knows what purity means, then what are all those tortured discussions on whether Haskell is pure or not about? ...

They're about what purity means!