I have a question about the best way to go about the following
I Have a class B, I have a combinator on B,
let foo : B -> int.
I want the class B to have the combinator encapsulated as a method, so I add it with a type extension.
I then later on realize that foo is quite expensive and want to cache it's result with lazy evaluation
So I add a huge clutch to the system by passing the combinator as a function to the constructor and then initializing a field with foo = lazy(foo self) in the constructor.
i.e.
type foo =
class
val x : int Lazy
new (comb) as self = {x=lazy(comb self);}
end
let something (x:foo) = 1
type foo with
new() = foo(something)
this obviously feels wrong
the two options I see for fixing this are 1, make an interface and have foo inherit that interface, 2, make everything a static method and then make combinators out of those static methods(sort of the opposite of attaching them to classes...)
Neither of these are hugely appealing and I was wondering if I missed option 3
Oh, and I haven't been able to get let rec and to work quite right with this, nor would i really want to as "something" in the above statement depends on a function that depends on a function that depends on a function(3 deep).
any advice would be appreciated
I don't think there is anything wrong with your current design. The key point is that if you define the type Foo as well as the extension to the type in a same file (and the same module), then F# will combine the two parts of the definition into a single .NET type. So, the fact that it is defined in two separate parts is just an implementation detail.
If you don't want to expose the constructor that takes the combinator, you can mark it as private. Together with a few additional changes (i.e. use implicit constructor syntax), the snippet would look like this:
type Foo private (comb) as self =
let x : Lazy<int> = lazy comb self
let something (x:Foo) = 1
type Foo with
new() = Foo(something)
If you want to keep something as a separate function, then this is a fine solution. Many numeric types in the F# PowerPack follow this pattern (see for example definition of complex numbers)
I don't quite grok what you're after, but I think this may help:
type foo(comb) as self =
let x = lazy(comb self)
static member something (x:foo) = 1
new() = foo(foo.something)
A type can be recursive with its own static member, so this is a simpler way to write your code.
Related
Given an interface that has a FSharp style function.
type IUseless =
abstract member Listify: string -> int -> string list
How would you mock the function?
let substitute = NSubstitute.Substitute.For<IUseless>()
substitute.Listify.Returns(??? what would even go here ???)
I wouldn't expect to be able to mock it like a normal method, or a value that contains a function (although that's sort of what it represents).
So I'm curious if anyone has successfully mocked an FSharp function with a typical .NET mocking library.
First: yes, you can totally mock this like a normal method:
let substitute = NSubstitute.Substitute.For<IUseless>()
(substitute.Listify "abc" 5).Returns ["whatevs"]
This works, because F# compiles this definition like a normal .NET method, despite the curried syntax. This is done partly for interop and partly for performance.
But second: if I were you, I would rather skip the whole NSubstitute business altogether and use inline interface implementation instead:
let substitute = { new IUseless with member x.Listify a b = ["whatevs"] }
This is cleaner, better typechecked, and a lot faster at runtime.
I have an F# class that uses the following to declare, but not initialize class members:
[<DefaultValue>] val mutable myVariable : myType
How can I check in the code whether this value has been initialized? I tried:
x.myVariable = null
but that doesn't seem to work.
From your description, it is a bit hard to say what you are actually trying to achieve - using both uninitialized values and inheritance is generally not the preferred way of doing things in F# (but they are both sometimes necessary for interoperability with .NET), so if you follow this direction, you might not be getting that many advantages from using F#.
Wouldn't the following work for you instead? The idea is that we define a base class that takes the value of the private thing through a constructor:
type Base(myThing : Random) =
member x.MyThing = myThing
And an inherited class can then provide a value, but also access it using a member:
type MySubclass() =
inherit Base(new Random(0))
member x.Next() =
x.MyThing.Next()
type Tag(Kids) =
member this.Kids = Kids
static member (-) (this: Tag, that: list<obj>) =
Tag(that::this.Kids)
The point of this code is to construct a new object based on an existing one, but modifying one (or more) fields on the object. Basically, it is a proxy for a thing.setX(...) mutator method, except using immutable data.
It looks incredibly ugly and verbose, and I'm sure there must be a better way of doing it while maintaining the immutability of the data, but I haven't figured out how. Is there some syntactically nice way of doing this?
EDIT: Just for clarity, I also have other classes in the hierarchy:
type HTMLTag(s, Classes, Kids, Styles) =
inherit Tag(Kids)
member this.NominalTag = s
member this.Classes = Classes
member this.Styles: list<String * String> = Styles
static member (-) (this: HTMLTag, that: list<obj>) =
HTMLTag(this.NominalTag, this.Classes, that::this.Kids, this.Styles)
Apart from it being very verbose, the - function's "copy with modification" thing is completely non-generic: even though I am doing the same thing each time (copy w/ modification to same variable) I have to rewrite the whole thing all over again, which isn't very nice.
One thing I find very annoying is that this would be very simple with mutation:
static member (-) (this: Tag, that: list<obj>) =
this.Kids = that :: this.Kids
this
But I'm trying to keep everything immutable as far as possible
Copy and update record expressions[MSDN] are meant to handle this exact case. If you can use a record type instead you can do
type Tag =
{ NominalTag : obj
Classes : obj
Kids : list<obj>
Styles : list<String * String> }
static member (-) (this: Tag, that: list<obj>) =
{ this with Kids = this.Kids # that }
The compiled forms of this code and yours are virtually identical.
Incidentally, it's odd that the (-) operator is being used to append...but I presume this is a contrived case.
UPDATE
Now that you've updated your question I'm confused about what you want to do. If you want to return a new object I don't see how mutation helps you.
A more functional approach (vs inheritance) is to separate your data and behaviors, the data being records and the behaviors functions grouped within a module. If you want behavior to be shared across types, use interfaces. Records can implement interfaces.
Let's say I have a function which does something pretty complicated and it is implemented with the help of subfunctions. To make things easier, instead of tuples I would like to use some intermediate structures which are private to the implementation of this function.
I don't want the declaration of these structures to leak outside. So I want something like this:
let someComplexFun p =
type SomeRecord = {i:int; x:int; y:int;}
type SomeOtherRecord = {...}
let innerFunctionA (x:SomeRecord) = ...
let innerFunctionB (x:SomeOtherRecord) = ...
...
I tried it but of course the compiler doesn't let me do this. I looked at the documentation and I can't see anywhere quickly that the types must be declared at the module level.
In LISP for example, it seems that it's all entirely legal, e.g.:
(defun foo (when)
(declare (type (member :now :later) when)) ; Type declaration is illustrative and in this case optional.
(ecase when
(:now (something))
(:later (something-else))))
So, am I missing something? Is this possible if F# at all?
To verify that this is not allowed according to the specification, take a look at the grammar of F# expressions in the specification: Section 6: Expressions. It lists various constructs that can be used in place of expr and none of them is a type declaration type-defn (described in Section 8: Type Declarations).
The (simplified) syntax for function declarations is let ident args = expr, so the body has to be an expression (and you cannot declare types inside expressions).
Types can only be declared at module or namespace scope in F#.
(You can use access modifiers like internal or signature files to hide types from other components.)
What Brian said.
heres a link to some more information http://www.ctocorner.com/fsharp/book/ch17.aspx
I know I must be missing something really obvious here. B.GetInstance().Call() generates the error: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.
I'm using v1.9.9.9.
type A() =
member x.Call() = B.GetInstance().Call()
and B() =
static member GetInstance() = new B()
member x.Call() = ()
I just discovered that this works: (B.GetInstance() :> B).Call()
Any idea why the cast is necessary?
Frequently when you've got a recursive set of methods whose types to infer, F# needs help. A more pleasant alternative would be to annotate the definition of B.GetInstance:
type A() =
member x.Call() = B.GetInstance().Call()
and B() =
static member GetInstance() : B = new B()
member x.Call() = ()
I believe that the reason you run into this problem is that F# tries to solve all inferred types on all methods in A and B simultaneously (because they are defined as mutually recursive types), and this leads to problems, but perhaps someone from the F# team will weigh in.
The quick summary is that in a recursive group (e.g. members in one type, or members of recursive types like we have here) F# reads the declarations in left-to-right top-to-bottom order, followed by the definitions in left-to-right top-to-bottom order. So in this instance when it reaches the definition of A.Call, it has not yet read the definition of B.GetInstance and therefore does not (yet!) know that the return type of GetInstance will be B.
Keith's answer nails it for this situation, you can provide a type annotation to specify the return type of GetInstance in its declaration.
See
Forcing F# type inference on generics and interfaces to stay loose
for a deep discussion of what's going on here.
Note also that in your original attempt, you don't need to "cast" (the potentially dynamic operation, using :>), instead you can just "annotate" (statically declare a type, using :) to get it to compile. But makes more sense to put the type annotation in the method declaration for GetInstance (generally, prefer addition annotations to method signatures instead of arbitrary places inside bodies).