I was reading up on XQuery (from this link: [1]). And LINQ (more accurately Query Expressions) seems to be an almost carbon-copy of it. So I'm wondering if Microsoft actually just copied XQuery and threw some syntactic sugar over it. I would say inspired but it doesn't seem strong enough a description.
An example:
// LINQ - Query Expression
var winners = from r in racers
where r.Wins > 3
orderby r.Wins descending
select r;
// XQuery
for $racer in doc("racers.xml")/Racers/Racer
where $racer/Wins > 3
order by $racer/Wins descending
return $racer
No, LINQ was not copied from XQuery.
As Eric Lippert indicates in this post:
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.
Related
UPDATE 8/31/2011
Guillaume Laforge has nearly done it:
http://gaelyk.appspot.com/tutorial/app-engine-shortcuts#query
Looks like he's doing an AST transform to pull off the:
alias as Entity
bit. Cool stuff, Groovy 1.8 + AST transform = LINQ-esque queries on the JVM. GL's solution needs more work as far as I can see to pull off full query capabilities (like sub queries, join using(field) syntax and the like), but for his Gaelyk project apparently not necessary.
EDIT
As a workaround to achieving pure LINQ syntax, I have decided to def the aliases. Not a huge deal, and removes a major hurdle that would likely require complex AST transforms to pull off.
So, instead of:
from c as Composite
join t as Teams
...
I now define the aliases (note: need to cast to get auto complete on fields):
def(Teams t,Composite c,Schools s) = [Teams.new(),Composite.new(),Schools.new()]
and use map syntax for from, join, etc.
from c:Composite
join t:Teams
...
To solve issue #2 (see original below), add instance level getProperty methods to each pogo alias (whose scope is limited to ORM closure in which it is called, nice). We just return the string property name as we're building an sql statement.
[t,c,s].each{Object o-> o.metaClass.getProperty = { String k-> k } }
Making "good" progress ;-)
Now to figure out what to do about "=", that is tricky since set property is void. May have to go with eq, neq, gt, etc. but would really prefer the literal symbols, makes for closer-to-sql readability.
If interested, LINQ is doing quite a bit behind the scenes. Jon Skeet (praise his name) has a nice SO reply:
How LINQ works internally?
ORIGINAL
Have been checking out LINQ, highly impressed.
// LINQ example
var games =
from t in Teams
from g in t.Games
where g.gameID = 212
select new { g.gameDate,g.gameTime };
// Seeking Groovy Nirvana
latest { Integer teamID->
from c as Composite
join t as Teams
join s as Schools on ( schoolID = {
from Teams
where t.schoolID = s.schoolID } )
where t.teamID = "$teamID"
select c.location, c.gameType, s.schoolName
group c.gameID
order c.gameDate, c.gameTime
}
The proposed Groovy version compiles fine and if I def the aliases c,t,s with their corresponding POGO, I get strong typed IDE auocomplete on fields, nice. However, nowhere near LINQ, where there are no (visible) variable definitions other than the query itself, totally self contained and strongly typed, wow.
OK, so can it be done in Groovy? I think (hope) yes, but am hung up on 2 issues:
1) How to implicitly populate alias variable without def'ing? Currently I am overriding asType() on String so in "from c as Composite", c gets cast to Composite. Great, but the IDE "thinks" that in closure scope undefined c is a string and thus no autocomplete on POGO fields ;-(
2) Since #1 is not solved, I am def'ing the aliases as per above so I can get autocomplete. Fine, hacked (compared to LINQ), but does the trick. Problem here is that in "select c.location, c.gameType...", I'd like the fields to not be evaluated but simply return "c.location" to the ORM select method, and not null (which is its default value). getProperty() should work here, but I need it to only apply to pogo fields when called from ORM scope (e.g. orm field specific methods like select, order, group, etc.). Bit lost there, perhaps there's a way to annotate orm methods, or only invoke "special" pogo getProperty via orm method calls (which is the closure's delegate in above nirvana query).
Should point out that I am not looking to create a comprehensive LINQ for Groovy, but this one particular subset of LINQ I would love to see happen.
One of the biggest reasons for Guillaume to use an AST transform is because of the problem with "=". Even if you use == for the compare as normally done in Groovy, from the compareTo method that is called for it, you cannot make out a difference between ==, !=, <=, >=, <, >. There are two possible paths for this in later versions of Groovy that are in discussion. One is to use for each of those compares a different method, the other is to store a minimal AST which you can access at runtime. This goes in the direction of C# and is a quite powerful tool. The problem is more about how to do this efficiently.
Why to use lambda expression when we have LINQ queries just to shorten the length of code , increase the speed of development or is there any other reason which can only be achieved by Lambda expression & not by LINQ queries.
Query expressions only cover a small subset of the LINQ operators, and are only applicable when you have the actual expression involved to hand, rather than (say) having a Func<T, bool> to act as the predicate, in which case things become ugly. So instead of writing:
Func<Foo, bool> predicate = ...; // Get predicate from somewhere
var query = from x in collection
where predicate(x)
select x;
I'd much rather write:
Func<Foo, bool> predicate = ...; // Get predicate from somewhere
var query = collection.Where(predicate);
There are various other cases where using non-query expression syntax is simpler, particularly if your query only uses a single operator.
Query expressions are effectively translated into non-query expressions, so anything you can do in query expressions can be expressed in non-query expressions. Use query expressions where they make the code simpler and more readable; don't use them where they don't.
I have more information about how query expressions work in a blog post that you may be interested in.
any other reason which can only be achieved by Lambda expression & not by LINQ queries.
There are some LINQ extension methods which do not have counterparts in LINQ query expressions, and will require the use of Lambda Expressions. A good example is Enumerable.ToLookup - if you want to create an ILookup, you need to use lambdas to generate this.
What are the pros and cons of LINQ operators and LINQ methods? Does one have limitations or added capabilities the other does not?
The term "operator" in LINQ isn't the same as "operator" in the normal sense of C# language operators, (+, && etc). The LINQ standard query operators are just the LINQ methods which are expected to be available where possible through most providers (and the ones made available through LINQ to Objects in particular).
Were you actually asking about the pros and cons of using query expressions like this:
var query = from item in source
where item.SomeProperty == 5
select item.OtherProperty;
vs the "fluent interface" or "dot notation" of normal extension method calls:
var query = source.Where(item => item.SomeProperty == 5)
.Select(item => item.OtherProperty);
? If so, I can write about that in more detail, but basically:
There'll generally be no performance difference, as query expressions are effectively translated by the compiler into the latter syntax. Occasionally there are overloads you can use from dot notation which aren't available via query expressions, and you may be able to make things more efficient that way.
Operators which use multiple delegates (e.g. Join, GroupBy) tend to be more readable in query expression syntax
If your query would use lots of transparent identifiers in query expression syntax, the dot notation is likely to be ugly
If your query is very simple (just a Where and/or Select) then dot notation is likely to be simpler
If you're using methods which are unsupported by query expressions (e.g. Count(), ToList() etc) then having to bracket the query expression can be ugly
I love using LINQ in .NET, but I want to know how that works internally?
It makes more sense to ask about a particular aspect of LINQ. It's a bit like asking "How Windows works" otherwise.
The key parts of LINQ are for me, from a C# perspective:
Expression trees. These are representations of code as data. For instance, an expression tree could represent the notion of "take a string parameter, call the Length property on it, and return the result". The fact that these exist as data rather than as compiled code means that LINQ providers such as LINQ to SQL can analyze them and convert them into SQL.
Lambda expressions. These are expressions like this:
x => x * 2
(int x, int y) => x * y
() => { Console.WriteLine("Block"); Console.WriteLine("Lambda"); }
Lambda expressions are converted either into delegates or expression trees.
Anonymous types. These are expressions like this:
new { X=10, Y=20 }
These are still statically typed, it's just the compiler generates an immutable type for you with properties X and Y. These are usually used with var which allows the type of a local variable to be inferred from its initialization expression.
Query expressions. These are expressions like this:
from person in people
where person.Age < 18
select person.Name
These are translated by the C# compiler into "normal" C# 3.0 (i.e. a form which doesn't use query expressions). Overload resolution etc is applied afterwards, which is absolutely key to being able to use the same query syntax with multiple data types, without the compiler having any knowledge of types such as Queryable. The above expression would be translated into:
people.Where(person => person.Age < 18)
.Select(person => person.Name)
Extension methods. These are static methods which can be used as if they were instance methods of the type of the first parameter. For example, an extension method like this:
public static int CountAsciiDigits(this string text)
{
return text.Count(letter => letter >= '0' && letter <= '9');
}
can then be used like this:
string foo = "123abc456";
int count = foo.CountAsciiDigits();
Note that the implementation of CountAsciiDigits uses another extension method, Enumerable.Count().
That's most of the relevant language aspects. Then there are the implementations of the standard query operators, in LINQ providers such as LINQ to Objects and LINQ to SQL etc. I have a presentation about how it's reasonably simple to implement LINQ to Objects - it's on the "Talks" page of the C# in Depth web site.
The way providers such as LINQ to SQL work is generally via the Queryable class. At their core, they translate expression trees into other query formats, and then construct appropriate objects with the results of executing those out-of-process queries.
Does that cover everything you were interested in? If there's anything in particular you still want to know about, just edit your question and I'll have a go.
LINQ is basically a combination of C# 3.0 discrete features of these:
local variable type inference
auto properties (not implemented in VB 9.0)
extension methods
lambda expressions
anonymous type initializers
query comprehension
For more information about the journey to get there (LINQ), see this video of Anders in LANGNET 2008:
http://download.microsoft.com/download/c/e/5/ce5434ca-4f54-42b1-81ea-7f5a72f3b1dd/1-01%20-%20CSharp3%20-%20Anders%20Hejlsberg.wmv
In simple a form, the compiler takes your code-query and converts it into a bunch of generic classes and calls. Underneath, in case of Linq2Sql, a dynamic SQL query gets constructed and executed using DbCommand, DbDataReader etc.
Say you have:
var q = from x in dc.mytable select x;
it gets converted into following code:
IQueryable<tbl_dir_office> q =
dc.mytable.Select<tbl_dir_office, tbl_dir_office>(
Expression.Lambda<Func<mytable, mytable>>(
exp = Expression.Parameter(typeof(mytable), "x"),
new ParameterExpression[] { exp }
)
);
Lots of generics, huge overhead.
Basically linq is a mixture of some language facilities (compiler) and some framework extensions. So when you write linq queries, they get executed using appropriate interfaces such as IQuerable. Also note that the runtime has no role in linq.
But it is difficult to do justice to linq in a short answer. I recommend you read some book to get yourself in it. I am not sure about the book that tells you internals of Linq but Linq in Action gives a good handson about it.
For example:
m_lottTorqueTools = (From t In m_lottTorqueTools _
Where Not t.SlotNumber = toolTuple.SlotNumber _
And Not t.StationIndex = toolTuple.StationIndex).ToList
What algorithm occurs here? Is there a nested for loop going on in the background? Does it construct a hash table for these fields? I'm curious.
Query expressions are translated into extension method calls, usually. (They don't have to be, but 99.9% of queries use IEnumerable<T> or IQueryable<T>.)
The exact algorithm of what that method does varies from method to method. Your sample query wouldn't use any hash tables, but joins or grouping operations do, for example.
The simple Where call translates to something like this in C# (using iterator blocks, which aren't available in VB at the moment as far as I'm aware):
public static IEnumerable<T> Where(this IEnumerable<T> source,
Func<T, bool> predicate)
{
// Argument checking omitted
foreach (T element in source)
{
if (predicate(element))
{
yield return element;
}
}
}
The predicate is provided as a delegate (or an expression tree if you're using IQueryable<T>) and is called on each item in the sequence. The results are streamed and execution is deferred - in other words, nothing happens until you start asking for items from the result, and even then it only does as much as it needs to in order to provide the next result. Some operators aren't deferred (basically the ones which return a single value instead of a sequence) and some buffer the input (e.g. Reverse has to read to the end of the sequence before it can return any results, because the last result it reads is the first one it has to yield).
It's beyond the scope of a single answer to give details of every single LINQ operator I'm afraid, but if you have questions about specific ones I'm sure we can oblige.
I should add that if you're using LINQ to SQL or another provider that's based on IQueryable<T>, things are rather different. The Queryable class builds up the query (with the help of the provider, which implements IQueryable<T> to start with) and then the query is generally translated into a more appropriate form (e.g. SQL) by the provider. The exact details (including buffering, streaming etc) will entirely depend on the provider.
LINQ in general has a lot going on behind the scenes. With any query, it is first translated into an expression tree using an IQueryableProvider and from there the query is typically compiled into CIL code and a delegate is generated pointing to this function, which you are essentially using whenever you call the query. That's an extremely simplified overview - if you want to read a great article on the subject, I recommend you look at How LINQ Works - Creating Queries. Jon Skeet also posted a good answer on this site to this question.
What happens not only depends on the methods, but also depends on the LINQ provider in use. In the case where an IQueryable<T> is being returned, it is the LINQ provider that interprets the expression tree and processes it however it likes.