typeclasses | Future C implementation of Rust-like trait objects
kandi X-RAY | typeclasses Summary
kandi X-RAY | typeclasses Summary
This is an idea of how Rust-like trait objects could be implemented in C++ using static reflection, code injection and metaclasses. This solution is very succinct, requiring no boilerplate or arcane tricks. It has been tested against the Clang fork at 02eaac5aa06dfa0d19de95270bccd6311f11f5ba.
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QUESTION
For me, OOP was all about Autonomy which is achieved by two main concepts: (1) Encapsulation and (2) Message Passing.
- Concepts (like subtyping, late binding, multiple dispatch, interfaces and implementations, prototype, access modifier, constructor and destructor, ...) are mechanisms to support encapsulation and message passing.
- Concepts (like responsibility, open-close principle, dependency injection, creation-usage separation, ...) are disciplines to design good objects.
And now I'm learning FP and want to figure out:
- FP is all about
[MASK]which is achieved by main concept(s): (1) Referential Transparency, and[MASK]* - Is concepts (like immutability, closure and higher-order functions, continuations, algebraic data types, categorical typeclasses or traits, ...) are mechanisms to support those main concepts?
- And what are important design disciplines for good functions? I think data-behavior separation would be one of them but not sure...
Can you share your insights on FP paradigm? Thank you!
...ANSWER
Answered 2022-Feb-27 at 07:53Most of the characteristics listed for OOP in the OP apply to FP as well - perhaps with the exception of inheritance.
But even so, some functional-first languages (e.g. F#) support inheritance, and some object-oriented languages don't have inheritance.
There's no single, accepted definition of exactly what constitutes OOP, and the same is true for FP. I discuss this in my article Functional architecture: a definition, where I also posit that the defining characteristic of FP is referential transparency.
Many of the other well-known traits of FP (immutability, higher-order functions, closures, etc.) stem directly or indirectly from the emphasis on referential transparency. Some typical FP trappings like monads could also be said to originate from referential transparency, but mostly in the sense that they are successful solutions to some of the problems that arise from referential transparency (e.g. hos to maintain state while performing a complex calculation, which can be addressed by the State monad).
Still, there are disparate takes on FP. Just compare Clojure to Haskell. These languages are both, by their communities, considered functional, and still, they are very different. Many of the characteristics suggested in the OP (type classes, algebraic data types) are typical for Haskell, while not really a thing in Clojure, because Clojure is dynamically typed.
Clojure, on the other hand, emphasises typical Lisp-like properties like homoiconicity and macros, and its community embrace dynamic typing. (Read this, however, with the caveat that I know Haskell much better than I know Clojure.)
QUESTION
Haskell typeclasses often come with laws; for instance, instances of Monoid are expected to observe that x <> mempty = mempty <> x = x.
Typeclass laws are often written with single-equals (=) rather than double-equals (==). This suggests that the notion of equality used in typeclass laws is something other than that of Eq (which makes sense, since Eq is not a superclass of Monoid)
Searching around, I was unable to find any authoritative statement on the meaning of = in typeclass laws. For instance:
- The Haskell 2010 report does not even contain the word "law" in it
- Speaking with other Haskell users, most people seem to believe that
=usually means extensional equality or substitution but is fundamentally context-dependent. Nobody provided any authoritative source for this claim. - The Haskell wiki article on monad laws states that
=is extensional, but, again, fails to provide a source, and I wasn't able to track down any way to contact the author of the relevant edit.
The question, then: Is there any authoritative source on or standard for the semantics for = in typeclass laws? If so, what is it? Additionally, are there examples where the intended meaning of = is particularly exotic?
(As a side note, treating = extensionally can get tricky. For instance, there is a Monoid (IO a) instance, but it's not really clear what extensional equality of IO values looks like.)
ANSWER
Answered 2022-Feb-24 at 22:30Typeclass laws are not part of the Haskell language, so they are not subject to the same kind of language-theoretic semantic analysis as the language itself.
Instead, these laws are typically presented as an informal mathematical notation. Most presentations do not need a more detailed mathematical exposition, so they do not provide one.
QUESTION
I need to implement the cats Show instances for an enum which express the basic operators.
ANSWER
Answered 2022-Feb-17 at 06:48implicit val minusShow: Show[Expr.Minus] = Show.show(
minus => show"${minus.left} - ${minus.right}"
)
QUESTION
Normally in Haskell, tuples of length one aren't allowed (AFAIK). However, when messing with Template Haskell, I got this:
...ANSWER
Answered 2022-Feb-03 at 21:48There's a Unit type defined in GHC.Tuple.
Quoting the source:
QUESTION
I have a typeclass with a default implementation, and would like to provide a simple way to derive the typeclass if a user wants to use their custom monad.
Here's a solution someone else provided me:
...ANSWER
Answered 2022-Jan-01 at 21:35As the error message says, your associated type FooCtx depends only on ctx, but not on m, thus creating a potential for ambiguity like this:
QUESTION
I have been reading Bruno's TypeClasses paper and he mentioned that implicits in the argument list are projected/propagated into the implicit scope. I followed the example with this code:
...ANSWER
Answered 2021-Dec-31 at 05:20Recall that the value of an implicit parameter is determined at the call site. That's why...
QUESTION
If I compile the following source file with ghc -Wall:
ANSWER
Answered 2021-Nov-30 at 15:34In Haskell, numeric literals have a polymorphic type
QUESTION
I'm new to Haskell. Concepts such as monad and monoid, and their corresponding typeclasses, are very interesting but highly abstract and distant. I wonder how those advanced concepts can make things easier to implement. Some self-contained concrete examples would be nice to see.
...ANSWER
Answered 2021-Nov-25 at 18:56You get a way of understanding your problem domain in terms of types and type classes. Here is what I can gather from looking at V3 and its instances, without thinking about what the type is used to represent (3D vector).
QUESTION
I was trying to find the source for a certain kind of inlining that happens in GHC, where a function that is passed as an argument to another function is inlined. For example, I may write a definition like the following (using my own List type to avoid rewrite rules):
...ANSWER
Answered 2021-Nov-25 at 10:34The optimization is called "call-pattern specialization" (a.k.a. SpecConstr) this specializes functions according to which arguments they are applied to. The optimization is described in the paper "Call-pattern specialisation for Haskell programs" by Simon Peyton Jones. The current implementation in GHC is different from what is described in that paper in two highly relevant ways:
- SpecConstr can apply to any call in the same module, not just recursive calls inside a single definition.
- SpecConstr can apply to functions as arguments, not just constructors. However, it doesn't work for lambdas, unless they have been floated out by full laziness.
Here is the relevant part of the core that is produced without this optimization, using -fno-spec-constr, and with the -dsuppress-all -dsuppress-uniques -dno-typeable-binds flags:
QUESTION
When implementing Typeclasses for our types, we can use different syntaxes (an implicit val or an implicit object, for example). As an example:
A Typeclass definition:
...ANSWER
Answered 2021-Nov-24 at 11:26As far as I know the differences would be:
objects have different initialization rules - quite often they will be lazily initialized (it doesn't matter if you don't perform side effects in constructor)- it would also be seen differently from Java (but again, you probably won't notice that difference in Scala)
object Xwill have a typeX.typewhich is a subtype of whateverXextends or implements (but implicit resolution would find that it extends your typeclass, though perhaps with a bit more effort)
So, I wouldn't seen any difference in the actual usage, BUT the implicit val version could generate less JVM garbage (I say garbage as you wouldn't use any of that extra compiler's effort in this particular case).
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