Haskell和LINQ之间有什么联系吗？

Question

I wrote some queries in C# using LINQ. After a while, I started using Haskell a little bit, which is a functional programming language (a not so popular one), and for me it seems that both of them are almost the same thing. But I am unsure about this. Please, if someone has used them more than me, could they tell me if they are almost the same thing regarding principles in programming ?

Also, could LINQ be considered functional programming ?

Thanks.

Answer 1

for me it seems that both of them are almost the same thing. But I am unsure about this. Please, if someone has used them more than me, could they tell me if they are almost the same thing regarding principles in programming ?

Yes, the design of LINQ query comprehensions was heavily influenced by the design of Haskell. Haskell expert Erik Meijer was on the C# language design committee when we designed LINQ; his insights were very valuable. (I joined the design team at the end of this process, so unfortunately I did not get to participate in all the interesting twists and turns the design went through over the years; it started being much more traditional OO than it ended up!)

If you've recently done a serious exploration into Haskell then you're probably becoming familiar with the idea of a monad. The LINQ syntax is designed specifically to make operations on the sequence monad feel natural, but in fact the implementation is more general; what C# calls "SelectMany" is a slightly modified form of the "Bind" operation on an arbitrary monad. You can actually use query comprehension with any monad, as my colleague Wes describes here , but doing so looks pretty weird and I recommend against it in production code.

Also, could LINQ be considered functional programming ?

Yes, LINQ is heavily influenced by ideas from functional programming. It is designed to treat functions as first-class objects, to emphasize calculation over side effects, and so on.

Answer 2

It may be worthwhile to take a look at Erik Meijer Lecture . Here's the description for it.

We kick off C9 Lectures with a journey into the world of Functional Programming with functional language purist and high priest of the lambda calculus, Dr. Erik Meijer (you can thank Erik for many of the functional constructs that have shown up in languages like C# and VB.NET. When you use LINQ, thank Erik in addition to Anders).

Answer 3

It is true that C# has gained some aspects of functional programming. First and foremost is the lambda statement, implemented as an anonymous delegate. You now no longer need to define a method as belonging to an object, and you can define a method in a similar fashion as any other variable. This allows for many functional-type constructs:

var mult = (a,b)=>a*b;
var square = (a)=>Math.Pow(a,2);
var multandsquare = (a,b)=>square(mult(a,b));

//None of the above give a lick about what a and b really are, until...
multandsquare(5,3); //== 225

The basic paradigm of functional programming - that basically anything you can tell a computer to do can be told in terms of higher-order functions - can be applied to C# programs, especially now that C# actually has higher-order functions. The compiler will force you to have at least one class with at least one public main method (that's just how OO works), but from that point you can define pretty much everything only in terms of functions (def: a subclass of "methods" that take N parameters and produce 1 output without "side effects") instantiated as lambdas.

Linq does have some functional structure. Basically its method-chain paradigm is an example of monadic processing, which is how sequences of operations are structured through operation encapsulation in functional languages (and now in C#). Calling a Linq method on an IEnumerable returns another IEnumerable, which is really a different concrete class which contains a handle to the source Enumerable and some lambda to perform. You can replicate it in a functional language very simply; it's a tuple of the source (itself a tuple of the current element and everything else) and a function to perform that transforms the source tuple into the result, one element at a time (which in Linq, as it would be implemented in a functional language, is a nesting of some operation into a defined "rollup" function). Call a method that must produce an actual answer (a concrete typed result), and all these functions are evaluated sufficiently to produce the desired result.

Answer 4

Also could LINQ be considered functional programming?

LINQ implies a functional style in that it resembles, for instance, a SQL select, which is read-only in principle. However, because the LINQ clauses in .NET are implemented by plain-old CLR routines, there is nothing to prevent LINQ expressions from modifying state.

Haskell, on the other hand, is a "pure" functional language, so expressions cannot modify global state. Although it is possible to perform IO and graphics operations, these are carried out using a very different idiom from anything in .NET — including F#, which lets you drop into a procedural style more or less transparently.