using let inline and member constraints I'll be able to make duck typing for known members but what if I would like to define a generic function like so:
let duckwrapper<'a> duck = ...
with the signature 'b -> 'a and where the returned value would be an object that implemented 'a (which would be an interface) and forwarded the calls to duck.
I've done this in C# using Reflection.Emit but I'm wondering if F# reflection, quotations or other constructs would make it easier.
Any suggestions on how to accomplish this?
EDIT
after reading Tims answer I thought I'd give a bit more details
What I was thinking of when I wrote about using quotations to help was something like:
{new IInterface with member x.SayHello() = !!<# %expr #>}
!! being an operator translating the quotation to a function and %expr being the unit of work for the method. I'd be able to translate the expression to a function (I guess) but wouldn't know how to
of course this wouldn't do the trick completely either since IInterface would be 'a which is where I hope F# reflection might have some handy functions so that I could construct a type based on a type object and some function values
EDIT
As an update to Tomas Petricek answer I'll give some code to explain my needs
type SourceRole =
abstract transfer : decimal -> context
and context(sourceAccount:account, destinationAccount) =
let source = sourceAccount
let destination = destinationAccount
member self.transfer amount =
let sourcePlayer =
{new SourceRole with
member this.transfer amount =
use scope = new TransactionScope()
let source = source.decreaseBalance amount
let destination = destination.increaseBalance amount
scope.Complete()
context(source,destination)
}
sourcePlayer.transfer(amount)
which is a try at porting "the" textbook example of DCI in F#. The source and destination are DCI roles. It's the idea that any data object that adhere's to a specific contract can play those. In this case the contract is simple. source needs a memberfunction called decreaseBalance and destination needs a member function called increaseBalance.
I can accomplish that for this specific case with let inline and member constraints.
But I'd like to write a set of functions that given an interface and an object. In this case it could be source (as the object) and
type sourceContract =
abstract decreaseBalance : decimal -> sourceContract
as the type. The result would be an object of type sourceContract that would pipe method calls to a method with the same name on the source object.
F# reflection (Microsoft.FSharp.Reflection) is an F#-friendly wrapper around the plain System.Reflection APIs, so I don't think it would add anything here.
Quotations can't define new types: (you'd need to define a new type to do your interface-based duck typing)
> <# { new IInterface with member x.SayHello = "hello" } #>;;
<# { new IInterface with member x.SayHello = "hello" } #>;;
---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
stdin(7,4): error FS0449: Quotations cannot contain object expressions
> <# type Test() = class end #>;;
<# type Test() = class end #>;;
---^^^^
stdin(8,4): error FS0010: Unexpected keyword 'type' in quotation literal
Reflection.Emit is still the way to go with this.
Edit:
I hope F# reflection might have some handy functions so that I could construct a type based on a type object and some function values
I'm afraid it doesn't. Here's the documentation on F# reflection: http://msdn.microsoft.com/en-gb/library/ee353491.aspx
You can compile F# quotations using components from F# PowerPack. So I think you could use quotations to generate and execute code at runtime. If you write a quotation representing a function & compile it you'll get a function value that you could use to implement an interface. Here is a trivial example:
#r "FSharp.PowerPack.Linq.dll"
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Linq.QuotationEvaluation
// Create a part using "Expr." calls explicitly
let expr = Expr.Value(13)
// Create a part using quotation syntax
let expr2 = <# (fun x -> x * %%expr) #>
// Compile & Run
let f = expr2.Compile()()
f 10
You can mix quotation syntax and calls to Expr, which makes it easier to compose code from basic blocks. The compilation is a bit stupid (currently) so the generated code won't be as efficient as usual F# code (but you'll need to measure it in your case).
I'm not quite sure I understand what exactly are you trying to do, so if you can provide more details, I can give more specific answer.
Related
I want to parse and compile a function that I have written at runtime, for example I have the following string I generated at runtime:
let str = "fun x y z -> [x; y; z;]"
I am looking for something that will allow me to do something similar to:
let myfun = eval str
(* eval returns the value returned by the code in the string so myfun will
have the type: 'a -> 'a -> 'a -> 'a list*)
Is there a way to do that in OCaml? I came across Dynlink but I am looking for a simpler way to do it.
There is no easier solution than compiling the code and Dynlinking the resulting library.
Or equivalently, one can use the REPL, write the string to the file system and them load it with #use.
Depending on your precise use case, MetaOCaml might be an alternative.
Another important point is that types cannot depend on values in a non-dependently typed language. Thus the type of eval needs to be restricted. For instance, in the Dynlinking path, the type of dynamically linked functions will be determined by the type of the hooks used to register them.
as title, I have the code:
val description= mutableMapOf(
"father2" to "123",
"father" to mutableMapOf<String,String>())
description["father"]["child"] = "child value"
if i try this: description["father"]["child"]="child value"
I will get the error:
Unresolved reference. None of the following candidates is applicable because of receiver type mismatch:
public inline operator fun <K, V> MutableMap<String!, String!>.set(key: String!, value: String!): Unit defined in kotlin.collections
public inline operator fun kotlin.text.StringBuilder /* = java.lang.StringBuilder */.set(index: Int, value: Char): Unit defined in kotlin.text
how can i do?
This is about typing (or lack of it).
The description map is created from two pairs, one from a String to a String, and the other from a String to a MutableMap<String, String>. The keys are both String, so Kotlin will infer String for the type of key. But what about the values? They're two completely unrelated types, with no common superclass except Any. So the type of description is inferred to be MutableMap<String, Any>. (You can check that in e.g. the REPL.)
So, when you pull out a value, what can the compiler tell about it? Almost nothing. It's an Any, so the only thing you can be sure of is that it's not null.
That's why the compiler isn't treating that pulled-out value like a map; as far as it knows, it might not be a map! (We know — but only because we can see what the map was initialised with, and that it's not modified since then, nor is the reference shared with any other objects or threads that could modify it. But that's a relatively rare circumstance.)
Kotlin is a strongly-typed language, which means the compiler tracks the type of everything, and tries very hard not to let you do anything that could cause a runtime error. Your code, if it compiled, would risk a ClassCastException any time the data didn't exactly match what you expected, which is hardly a route to good programs.
So, what can you do?
The safest thing would be to change your program so that you don't have any maps with unknown types of values. Obviously, that depends on what your program is doing, so we can't make useful suggestions here. But if your description were a MutableMap<String, MutableMap<String, String>>, then the compiler would know exactly what type everything was, and your intended code would compile and run fine.
The other main alternative would be to check what the value is before trying to treat it like a map. One way to do this uses the as? safe-cast operator. That checks whether a value is of a given type; if so, it's cast to that type; otherwise, it returns null. You can then use the .? safe-call operator to call a method only if the value isn't null. Putting that together:
(description["father"] as? MutableMap<String, String>)?.put("child", "child value")
If the value is as you expect, that will work fine; if not, it will do nothing and continue.
That's rather long-winded, but it will compile and run safely. Alternatively, you could do the same thing in a more explicit way like this:
val child = description["father"]
if (child is MutableMap<String, String>))
child["child"] = "child value"
(Within the if, the compiler knows the type of child, and uses a a ‘smart cast’ to allow the putter call.)
That's even more long-winded, of course. But that's what you get from trying to work against the type system :-)
(Oh, and by the way, I think the usual terminology for recursive data structures is ‘parent’ and ‘child’; I've not seen ‘father’ used that way before.)
Try this:
description["father"]?.put("child", "child value")
My current solution:
val description:MutableMap<String,Any> = mutableMapOf(
"father" to "123"
)
val father2 = mutableMapOf<String,String>()
father2.put("child", "child value")
description["father2"]=father2
You can try this:
val description = mutableMapOf<String?,MutableMap<String,String>?>
("father" to mutableMapOf<String,String>())
description.get("father")?.put("Key","Value")
println(description) // {father={Key=Value}}
Given the following CRTP type in C#:
public abstract class DataProviderBase<TProvider>
where TProvider : DataProviderBase<TProvider> { }
How would I get its generic type definition in F#?
let typeDef = typedefof<DataProviderBase<_>>
yields the error:
Type constraint mismatch when applying the default type 'DataProviderBase<'a>' for a type inference variable. The resulting type would be infinite when unifying ''a' and 'DataProviderBase<'a>' Consider adding further type constraints
In C#, it would be:
var typeDef = typeof(DataProviderBase<>);
UPDATE
I found a workaround:
[<AbstractClass>]
type DummyProvider() =
inherit DataProviderBase<DummyProvider>()
let typeDef = typeof<DummyProvider>.BaseType.GetGenericTypeDefinition()
Is there another way to do it, without the extra type?
I think this is actually a very good question. I didn't find a better workaround for this.
You can slightly simplify your workaround by using typedefof like this:
let typeDef = typedefof<DataProviderBase<DummyProvider>>
TECHNICAL DETAILS
The problem is that F#'s typedefof<'T> is just an ordinary function that takes a type argument (unlike typeof in C#, which is an operator). In order to call it, you need to give it an actual type and the function will then call GetGenericTypeDefinition under the cover.
The reason why typedefof<option<_>> works is that F# specifies a default type as an argument (in this case obj). In general, F# chooses the less concrete type that matches the constraints. In your case:
DataProviderBase<_> will become DataProviderBase<DataProviderBase<_>> and so on.
Unless you define a new type (as in your workaround), there is no concrete type that could be used as a type argument of typedefof<...>. In this case, the defaulting mechanism simply doesn't work...
Let's say I have a Foo.fsx script that contains this code:
module Bar =
let foobar = "foo"+"bar"
open Bar
let a = System.Reflection.Assembly.GetExecutingAssembly()
let ty = a.GetType("Foo.Bar") // but it returns null here
How can I achieve that? Thanks!
This is a tricky question, because F# interactive compiles all types into types with some mangled names (and the executing assembly can contain multiple versions of the same type).
I managed to do it using a simple trick - you'd add an additional type to the module - then you can use typeof<..> to get information about this type. The type inside a module is compiled as a nested type, so you can get the name of the type representing the module from this (nested) type:
module Bar =
let foobar = "foo"+"bar"
type A = A
// Get type of 'Bar.A', the name will be something like "FSI_0001+Bar+A",
// so we remove the "+A" from the name and get "FSI_0001+Bar"
let aty = typeof<Bar.A>
let barName = aty.FullName.Substring(0, aty.FullName.Length - "+A".Length)
let barTy = aty.Assembly.GetType(barName)
// Get value of the 'foobar' property - this works!
barTy.GetProperty("foobar").GetValue(null, [| |])
You could probably simply search all types in the assembly looking for +Bar. That would work as well. The benefit of the trick above is that you get a reference to the specific version of the type (if you interactively run the code multiple times, you'll get a reference to the module corresponding to the current Bar.A type)
As a side-note, there were some discussions about supporting moduleof<Bar> (or something like that) in future versions of F# - this is a bit inelegant as it is not a real function/value like typeof, but it would very useful!
I've read through previous topics on closures on stackflow and other sources and one thing is still confusing me. From what I've been able to piece together technically a closure is simply the set of data containing the code of a function and the value of bound variables in that function.
In other words technically the following C function should be a closure from my understanding:
int count()
{
static int x = 0;
return x++;
}
Yet everything I read seems to imply closures must somehow involve passing functions as first class objects. In addition it usually seems to be implied that closures are not part of procedural programming. Is this a case of a solution being overly associated with the problem it solves or am I misunderstanding the exact definition?
No, that's not a closure. Your example is simply a function that returns the result of incrementing a static variable.
Here's how a closure would work:
function makeCounter( int x )
{
return int counter() {
return x++;
}
}
c = makeCounter( 3 );
printf( "%d" c() ); => 4
printf( "%d" c() ); => 5
d = makeCounter( 0 );
printf( "%d" d() ); => 1
printf( "%d" c() ); => 6
In other words, different invocations of makeCounter() produce different functions with their own binding of variables in their lexical environment that they have "closed over".
Edit: I think examples like this make closures easier to understand than definitions, but if you want a definition I'd say, "A closure is a combination of a function and an environment. The environment contains the variables that are defined in the function as well as those that are visible to the function when it was created. These variables must remain available to the function as long as the function exists."
For the exact definition, I suggest looking at its Wikipedia entry. It's especially good. I just want to clarify it with an example.
Assume this C# code snippet (that's supposed to perform an AND search in a list):
List<string> list = new List<string> { "hello world", "goodbye world" };
IEnumerable<string> filteredList = list;
var keywords = new [] { "hello", "world" };
foreach (var keyword in keywords)
filteredList = filteredList.Where(item => item.Contains(keyword));
foreach (var s in filteredList) // closure is called here
Console.WriteLine(s);
It's a common pitfall in C# to do something like that. If you look at the lambda expression inside Where, you'll see that it defines a function that it's behavior depends on the value of a variable at its definition site. It's like passing a variable itself to the function, rather than the value of that variable. Effectively, when this closure is called, it retrieves the value of keyword variable at that time. The result of this sample is very interesting. It prints out both "hello world" and "goodbye world", which is not what we wanted. What happened? As I said above, the function we declared with the lambda expression is a closure over keyword variable so this is what happens:
filteredList = filteredList.Where(item => item.Contains(keyword))
.Where(item => item.Contains(keyword));
and at the time of closure execution, keyword has the value "world," so we're basically filtering the list a couple times with the same keyword. The solution is:
foreach (var keyword in keywords) {
var temporaryVariable = keyword;
filteredList = filteredList.Where(item => item.Contains(temporaryVariable));
}
Since temporaryVariable is scoped to the body of the foreach loop, in every iteration, it is a different variable. In effect, each closure will bind to a distinct variable (those are different instances of temporaryVariable at each iteration). This time, it'll give the correct results ("hello world"):
filteredList = filteredList.Where(item => item.Contains(temporaryVariable_1))
.Where(item => item.Contains(temporaryVariable_2));
in which temporaryVariable_1 has the value of "hello" and temporaryVariable_2 has the value "world" at the time of closure execution.
Note that the closures have caused an extension to the lifetime of variables (their life were supposed to end after each iteration of the loop). This is also an important side effect of closures.
From what I understand a closure also has to have access to the variables in the calling context. Closures are usually associated with functional programming. Languages can have elements from different types of programming perspectives, functional, procedural, imperative, declarative, etc. They get their name from being closed over a specified context. They may also have lexical binding in that they can reference the specified context with the same names that are used in that context. Your example has no reference to any other context but a global static one.
From Wikipedia
A closure closes over the free variables (variables which are not local variables)
A closure is an implementation technique for representing procedures/functions with local state. One way to implement closures is described in SICP. I will present the gist of it, anyway.
All expressions, including functions are evaluated in an environement, An environment is a sequence of frames. A frame maps variable names to values. Each frame also has a
pointer to it's enclosing environment. A function is evaluated in a new environment with a frame containing bindings for it's arguments. Now let us look at the following interesting scenario. Imagine that we have a function called accumulator, which when evaluated, will return another function:
// This is some C like language that has first class functions and closures.
function accumulator(counter) {
return (function() { return ++counter; });
}
What will happen when we evaluate the following line?
accum1 = accumulator(0);
First a new environment is created and an integer object (for counter) is bound to 0 in it's first frame. The returned value, which is a new function, is bound in the global environment. Usually the new environment will be garbage collected once the function
evaluation is over. Here that will not happen. accum1 is holding a reference to it, as it needs access to the variable counter. When accum1 is called, it will increment the value of counter in the referenced environment. Now we can call accum1 a function with local state or a closure.
I have described a few practical uses of closures at my blog
http://vijaymathew.wordpress.com. (See the posts "Dangerous designs" and "On Message-Passing").
There's a lot of answers already, but I'll add another one anyone...
Closures aren't unique to functional languages. They occur in Pascal (and family), for instance, which has nested procedures. Standard C doesn't have them (yet), but IIRC there is a GCC extension.
The basic issue is that a nested procedure may refer to variables defined in it's parent. Furthermore, the parent may return a reference to the nested procedure to its caller.
The nested procedure still refers to variables that were local to the parent - specifically to the values those variables had when the line making the function-reference was executed - even though those variables no longer exist as the parent has exited.
The issue even occurs if the procedure is never returned from the parent - different references to the nested procedure constructed at different times may be using different past values of the same variables.
The resolution to this is that when the nested function is referenced, it is packaged up in a "closure" containing the variable values it needs for later.
A Python lambda is a simple functional-style example...
def parent () :
a = "hello"
return (lamda : a)
funcref = parent ()
print funcref ()
My Pythons a bit rusty, but I think that's right. The point is that the nested function (the lambda) is still referring to the value of the local variable a even though parent has exited when it is called. The function needs somewhere to preserve that value until it's needed, and that place is called a closure.
A closure is a bit like an implicit set of parameters.
Great question! Given that one of the OOP principles of OOP is that objects has behavior as well as data, closures are a special type of object because their most important purpose is their behavior. That said, what do I mean when I talk about their "behavior?"
(A lot of this is drawn from "Groovy in Action" by Dierk Konig, which is an awesome book)
On the simplest level a close is really just some code that's wrapped up to become an androgynous object/method. It's a method because it can take params and return a value, but it's also an object in that you can pass around a reference to it.
In the words of Dierk, imagine an envelope that has a piece of paper inside. A typical object would have variables and their values written on this paper, but a closure would have a list of instructions instead. Let's say the letter says to "Give this envelope and letter to your friends."
In Groovy: Closure envelope = { person -> new Letter(person).send() }
addressBookOfFriends.each (envelope)
The closure object here is the value of the envelope variable and it's use is that it's a param to the each method.
Some details:
Scope: The scope of a closure is the data and members that can be accessed within it.
Returning from a closure: Closures often use a callback mechanism to execute and return from itself.
Arguments: If the closure needs to take only 1 param, Groovy and other langs provide a default name: "it", to make coding quicker.
So for example in our previous example:
addressBookOfFriends.each (envelope)
is the same as:
addressBookOfFriends.each { new Letter(it).send() }
Hope this is what you're looking for!
An object is state plus function.
A closure, is function plus state.
function f is a closure when it closes over (captured) x
I think Peter Eddy has it right, but the example could be made more interesting. You could define two functions which close over a local variable, increment & decrement. The counter would be shared between that pair of functions, and unique to them. If you define a new pair of increment/decrement functions, they would be sharing a different counter.
Also, you don't need to pass in that initial value of x, you could let it default to zero inside the function block. That would make it clearer that it's using a value which you no longer have normal access to otherwise.