Re: [Haskell-cafe] Pure functional and pure logical language at the same time

Hello! Disclaimer: I have not worked in either of the languages I am talking about (Curry, Mercury). Curry is logical, but it uses different logic mechanisms than Prolog. Curry is using something called "narrowing" which is very different to Prolog's SLD-resolution. AFAICT narrowing (or needed narrowing) corresponds to lazy evaluation while SLD-resolution corresponds to strict evaluation. Mercury is another language to combine functional and logical features. But while Curry is Haskell-based with added mechanism of narrowing, Mercury is Prolog-based with Haskell-like type system. As Mercury is based on Prolog, there is an interesting part of a type system that corresponds to in/out parameters of a predicate. A predicate may also have multiple type signatures (since a some parameters may be in/out switched). The way Mercury treats IO in a pure way is also interesting (IIRC it uses something called unique parameter for the World). I see both languages as "pure". Both languages have pure functional parts (deterministic functions in Curry and function signatures in Mercury). I may be wrong, but it seems that of those two languages Mercury is a bit "more alive". Best, Nick 2015-01-28 21:37 GMT+03:00 Timotej Tomandl :

Second thing, is it purely logical? And do we get any advantage when reasoning?

2015-01-28 19:27 GMT+01:00 Andy Morris :

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Yes. It has monadic IO exactly like Haskell's.

On 28 Jan 2015, at 19:22, Timotej Tomandl wrote:

By pure logical I mean having features like in Pure Prolog. I am not sure if Curry is pure functional or not. Is it?

2015-01-28 19:09 GMT+01:00 Mathieu Boespflug <0xbadcode@gmail.com>:

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By "pure logical", do you mean "logically consistent"? Or do you mean a logic language in the style of Prolog but with no non-logical effects such as programmatic pruning of the search tree?

If the former, there are many languages in that fit the bill (or at least are believed to be consistent): Coq, Agda, HOL, ... The key difference between those languages and Haskell is that all functions must be provably total. See

https://uf-ias-2012.wikispaces.com/file/view/turner.pdf

from the father of a direct ancestor of Haskell, arguing for precisely this: a total functional programming language (not that I agree that such a language as proposed in the paper would be particularly useful).

If you mean the former, there are language that try to combine logical and functional programming, see e.g. Curry just mentioned in this thread a few minutes ago.

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This question was bugging me for quite a long time. Can we have a language which uses the functional logic while being both pure functional and

On 28 January 2015 at 18:22, Timotej Tomandl wrote: pure

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logical? Do we get any advantages from maintaining both both of this purities at the same time?

P.S.: I have feeling the answer is no, but I am not sure.

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