I'm trying to implement a typeprovider using the examples I found in different places.
What I want is to be able to create a constructor which assigns the provided arguments to the right properties. The result will have do the same thing as the type below.
type SomeType(arg1: int, arg2: string) =
member this.Arg1 = arg1
member this.Arg2 = arg2
I've tried different approaches, but I just can't get passed the first argument in the args list.
ProvidedConstructor(
parameters = parameters,
InvokeCode = (fun args -> ??)
What kind of code must be invoked to achieve this. Or do I have to take another approach?
What will be the underlying runtime representation of your provided type (the type you passed to the ProvidedTypeDefinition baseType argument)? The ProvidedConstructor InvokeCode is a function that takes a list of expressions and returns an expression of the underlying type. For example, if the runtime representation is a 2-element tuple, InvokeCode would be something like this:
InvokeCode = (fun [arg1;arg2] -> <## (%%arg1:int), (%%arg2:string) ##>)
Make sure to read this tutorial
Related
I am trying to develop F# type provider.
It provides some DTOs (with the structure described in some external document) and a set of methods for processing them. The processing algorithm is based on reflection, and I want to have a single quotation representing it.
Generally, this algorithm must pass all method call arguments to the already written function serialize: obj -> MySerializationFormat, storing all results in a list, so I getting a value of MySerializationFormat list.
Code sample below shows, how I tried to do that for first time:
let serialize (value: obj) = ...
let processingCode: Expr list -> Expr =
fun args ->
let serializeArgExpr (arg: Expr) = <# serialize %%arg} #>
let argsExprs = List.map serializeArgExpr args
let serializedArgList =
List.foldBack (fun head tail -> <# (%head) :: (%tail)#>) argsExprs <# [] #>
// futher processing
At that point I faced with exception: In function serializeArgExpr the actual type of value in arg: Expr may vary, it can be some primitive type (e.g string, int, float), or some provided type. The problem is %% operator treats that arg as an expression of the obj type. Type check is performed on that line in Microsoft.FSharp.Quotations.Patterns module, in function fillHolesInRawExpr.
So, as the actual type of my term not matched the treated type for "hole" in the quotation, it throws invalidArg.
I have tried several technics to avoid these exceptions with casting operations in my quotation, but they don't work. Then I found Expr.Coerce(source, target) function, which looks like solving my problem. I have changed the code of serializeArgExpr to something like that:
let serializeArgExpr (arg: Expr) =
let value' = Expr.Coerce(value, typeof<obj>)
<# serialize %%value' } #>
Then faced a new strange exception:
The design-time type (point to a code line that uses my processingCode) utilized by a type provider was not found in the target reference assembly set
For me, it seems that my problem is to cast the type of value in any input Expr to an obj type. Thank you for diving in and trying to help.
I was attempting to convert this to F# but I can't figure out what I'm doing wrong as the error message (in title) is too broad of an error to search for, so I found no resolutions.
Here is the code:
let getIP : string =
let host = Dns.GetHostEntry(Dns.GetHostName())
for ip in host.AddressList do
if ip.AddressFamily = AddressFamily.InterNetwork then
ip.ToString() // big fat red underline here
"?"
A for loop in F# is for running imperative-style code, where the code inside the for loop does not produce a result but instead runs some kind of side-effect. Therefore, the expression block in an F# for loop is expected to produce the type unit, which is what side-effect functions should return. (E.g., printfn "Something" returns the unit type). Also, there's no way to exit a for loop early in F#; this is by design, and is another reason why a for loop isn't the best approach to do what you're trying to do.
What you're trying to do is go through a list one item at a time, find the first item that matches some condition, and return that item (and, if the item is not found, return some default value). F# has a specialized function for that: Seq.find (or List.find if host.AddressList is an F# list, or Array.find if host.AddressList is an array. Those three functions take different input types but all work the same way conceptually, so from now on I'll focus on Seq.find, which takes any IEnumerable as input so is most likely to be what you need here).
If you look at the Seq.find function's type signature in the F# docs, you'll see that it is:
('T -> bool) -> seq<'T> -> 'T
This means that the function takes two parameters, a 'T -> bool and seq<'T> and returns a 'T. The 'T syntax means "this is a generic type called T": in F#, the apostrophe means that what follows is the name of a generic type. The type 'T -> bool means a function that takes a 'T and returns a Boolean; i.e., a predicate that says "Yes, this matches what I'm looking for" or "No, keep looking". The second argument to Seq.find is a seq<'T>; seq is F#'s shorter name for an IEnumerable, so you can read this as IEnumerable<'T>. And the result is an item of type 'T.
Just from that function signature and name alone, you can guess what this does: it goes through the sequence of items and calls the predicate for each one; the first item for which the predicate returns true will be returned as the result of Seq.find.
But wait! What if the item you're looking for isn't in the sequence at all? Then Seq.find will throw an exception, which may not be the behavior you're looking for. Which is why the Seq.tryFind function exists: its function signature looks just like Seq.find, except for the return value: it returns 'T option rather than 'T. That means that you'll either get a result like Some "ip address" or None. In your case, you intend to return "?" if the item isn't found. So you want to convert a value that's either Some "ip address or None to either "ip address" (without the Some) or "?". That is what the defaultArg function is for: it takes a 'T option, and a 'T representing the default value to return if your value is None, and it returns a plain 'T.
So to sum up:
Seq.tryFind takes a predicate function and a sequence, and returns a 'T option. In your case, this will be a string option
defaultArg takes a 'T option and a default value, and returns a normal 'T (in your case, a string).
With these two pieces, plus a predicate function you can write yourself, we can do what you're looking for.
One more note before I show you the code: you wrote let getIP : string = (code). It seems like you intended for getIP to be a function, but you didn't give it any parameters. Writing let something = (code block) will create a value by running the code block immediately (and just once) and then assigning its result to the name something. Whereas writing let something() = (code block) will create a function. It will not run the code block immediately, but it will instead run the code block every time the function is called. So I think you should have written let getIP() : string = (code).
Okay, so having explained all that, let's put this together to give you a getIP function that actually works:
let getIP() = // No need to declare the return type, since F# can infer it
let isInternet ip = // This is the predicate function
// Note that this function isn't accessible outside of getIP()
ip.AddressFamily = AddressFamily.InterNetwork
let host = Dns.GetHostEntry(Dns.GetHostName())
let maybeIP = Seq.tryFind isInternet host.AddressList
defaultArg maybeIP "?"
I hope that's clear enough; if there's anything you don't understand, let me know and I'll try to explain further.
Edit: The above has one possible flaw: the fact that F# may not be able to infer the type of the ip argument in isInternet without an explicit type declaration. It's clear from the code that it needs to be some class with an .AddressFamily property, but the F# compiler can't know (at this point in the code) which class you intend to pass to this predicate function. That's because the F# compiler is a single-pass compiler, that works its way through the code in a top-down, left-to-right order. To be able to infer the type of the ip parameter, you might need to rearrange the code a little, as follows:
let getIP() = // No need to declare the return type, since F# can infer it
let host = Dns.GetHostEntry(Dns.GetHostName())
let maybeIP = host.AddressList |> Seq.tryFind (fun ip -> ip.AddressFamily = AddressFamily.InterNetwork)
defaultArg maybeIP "?"
This is actually more idiomatic F# anyway. When you have a predicate function being passed to Seq.tryFind or other similar functions, the most common style in F# is to declare that predicate as an anonymous function using the fun keyword; this works just like lambdas in C# (in C# that predicate would be ip => ip.AddressFamily == AddressFamily.InterNetwork). And the other thing that's common is to use the |> operator with things like Seq.tryFind and others that take predicates. The |> operator basically* takes the value that's before the |> operator and passes it as the last parameter of the function that's after the operator. So foo |> Seq.tryFind (fun x -> xyz) is just like writing Seq.tryFind (fun x -> xyz) foo, except that foo is the first thing you read in that line. And since foo is the sequence that you're looking in, and fun x -> xyz is how you're looking, that feels more natural: in English, you'd say "Please look in my closet for a green shirt", so the concept "closet" comes up before "green shirt". And in idiomatic F#, you'd write closet |> Seq.find (fun shirt -> shirt.Color = "green"): again, the concept "closet" comes up before "green shirt".
With this version of the function, F# will encounter host.AddressList before it encounters fun ip -> ..., so it will know that the name ip refers to one item in host.AddressList. And since it knows the type of host.AddressList, it will be able to infer the type of ip.
* There's a lot more going on behind the scenes with the |> operator, involving currying and partial application. But at a beginner level, just think of it as "puts a value at the end of a function's parameter list" and you'll have the right idea.
In F# any if/else/then-statement must evaluate to the same type of value for all branches. Since you've omitted the else-branch of the expression, the compiler will infer it to return a value of type unit, effectively turning your if-expression into this:
if ip.AddressFamily = AddressFamily.InterNetwork then
ip.ToString() // value of type string
else
() // value of type unit
Scott Wlaschin explains this better than me on the excellent F# for fun and profit.
This should fix the current error, but still won't compile. You can solve this either by translating the C#-code more directly (using a mutable variable for the localIP value, and doing localIP <- ip.ToString() in your if-clause, or you could look into a more idiomatic approach using something like Seq.tryFind.
I'm trying to build my first toy-like Type Provider. What I'm trying to achieve is to have dynamically generated properties of dynamically generated types.
collection
|> getItems
|> Seq.map(fun mapItem ->
let nestedType = ProvidedTypeDefinition(assembly, ns, "MyNestedType", None)
let ctor = ProvidedConstructor(List.Empty)
nestedType.AddMember ctor
mapItem.Value
|> Seq.map(fun pair ->
ProvidedProperty(fst(pair), typeof<string>,
GetterCode = fun [_] -> <## snd(pair) ##>))
|> Seq.toList
|> nestedType.AddMembers
ProvidedProperty(mapItem.Key, nestedType,
GetterCode = fun [map] ->
// ?? Runtime Exception
let inst = nestedType.GetConstructors().[0].Invoke([||])
<## inst ##>
))
|> Seq.toList
|> ty.AddMembers
ty
How should I instantiate dynamically generated type ?
I'm assuming this is an erasing type provider (those are the easy ones, so they're better choice for getting started). If that's not the case, then disregard my answer.
In the GetterCode, you do not need to create instance of the nested provided type. You just need to create an instance of the type that it is erased to.
In your case, nestedType is erased to None and so the constructor just needs to create a System.Object value, so you should be able to use:
ProvidedProperty(mapItem.Key, nestedType,
GetterCode = fun [self] -> <## obj() ##>)
In reality, you'll probably want to erase to some type that lets you keep some data that the nested type is supposed to access. If the nested type was erased to, say, MyRuntimeType, you could then write:
let parameter = mapItem.WhateverYouWantHere
ProvidedProperty(mapItem.Key, nestedType,
GetterCode = fun [self] -> <## MyRuntimeType(parameter) ##>)
Note that I'm using let to capture the value of the primitive parameter type, so that the compiler can serialize the quotation (you cannot capture complex object types in a quotation).
What you're trying to do here is instantiate your type while building the provider, and then include that new instance in the body of the property. It should be abundantly clear that you can't instantiate the provided type before you've finished providing it.
What you really want to do is take your provided constructor and build a quotation that calls it. You can't have the compiler build the quotation for you, because in order for the compiler to compile the body of the quotation, it needs to "see" all types/methods/functions inside, and your type is not yet ready. But you can create the quotation manually by using the various constructors under Quotations.Expr. In this case, NewObject is suitable:
GetterCode = fun [map] -> Expr.NewObject (ctor, [])
I am running into difficulty with F# in numerous scenarios. I believe I'm not grasping some fundamental concepts. I'm hoping someone can track my reasoning and figure out the (probably many) things I'm missing.
Say I'm using Xunit. What I'd like to do is, provided two lists, apply the Assert.Equal method pairwise. For instance:
Open Xunit
let test1 = [1;2;3]
let test2 = [1;2;4]
List.map2 Assert.Equal test1 test2
The compiler complains that the function Equal does not take one parameter. As far as I can tell, shouldn't map2 be providing it 2 parameters?
As a sanity check, I use the following code in f# immediate:
let doequal = fun x y -> printf "result: %b\n" (x = y)
let test1 = [1;2;3]
let test2 = [1;2;4]
List.map2 doequal test1 test2;;
This seems identical. doequal is a lambda taking two generic parameters and returning unit. List.map2 hands each argument pairwise into the lambda and I get exactly what I expected as output:
result: true
result: true
result: false
So what gives? Source shows Xunit.Equal has signature public static void Equal<T>(T expected, T actual). Why won't my parameters map right over the method signature?
EDIT ONE
I thought two variables x and y vs a tuple (x, y) could construct and deconstruct interchangeably. So I tried two options and got different results. It seems the second may be further along than the first.
List.map2 Assert.Equal(test1, test2)
The compiler now complains that 'Successive arguments should be separated spaces or tupled'
List.map2(Assert.Equal(test1, test2))
The compiler now complains that 'A unique overload method could not be determined... A type annotation may be needed'
I think that part of the problem comes from mixing methods (OO style) and functions (FP style).
FP style functions have multiple parameters separated by spaces.
OO style methods have parens and parameters separated by commas.
Methods in other .NET libraries are always called using "tuple" syntax (actually subtly different from tuples though) and a tuple is considered to be one parameter.
The F# compiler tries to handle both approaches, but needs some help occasionally.
One approach is to "wrap" the OO method with an FP function.
// wrap method call with function
let assertEqual x y = Assert.Equal(x,y)
// all FP-style functions
List.map2 assertEqual test1 test2
If you don't create a helper function, you will often need to convert multiple function parameters to one tuple when calling a method "inline" with a lambda:
List.map2 (fun x y -> Assert.Equal(x,y)) test1 test2
When you mix methods and functions in one line, you often get the "Successive arguments should be separated" error.
printfn "%s" "hello".ToUpper()
// Error: Successive arguments should be separated
// by spaces or tupled
That's telling you that the compiler is having problems and needs some help!
You can solve this with extra parens around the method call:
printfn "%s" ("hello".ToUpper()) // ok
Or sometimes, with a reverse pipe:
printfn "%s" <| "hello".ToUpper() // ok
The wrapping approach is often worth doing anyway so that you can swap the parameters to make it more suitable for partial application:
// wrap method call with function AND swap params
let contains searchFor (s:string) = s.Contains(searchFor)
// all FP-style functions
["a"; "b"; "c"]
|> List.filter (contains "a")
Note that in the last line I had to use parens to give precedence to contains "a" over List.filter
public static void Equal<T>(T expected, T actual)
doesn't take two parameters - it takes one parameter, which is a tuple with two elements: (T expected, T actual).
Try this instead:
List.map2 Assert.Equal(test1, test2)
It's all there in the type signatures.
The signature for Assert.Equals is something along the lines of 'a * 'a -> unit. List.map2 expects a 'a -> 'b -> 'c.
They just don't fit together.
List.map2 (fun x y -> Assert.Equal(x,y)) test1 test2 - works because the lambda wrapping Equals has the expected signature.
List.zip test1 test2 |> List.map Assert.Equal - works because you now have a single list of tuples, and since List.map wants an 'a -> 'b function (where 'a is now a tuple), Assert.Equal is now fair game.
It's simply not true that two values and a tuple are implicitly interchangeable. At least not as far as F# the language is concerned, or the underlying IL representation is concerned. You can think that it's that way when you call into F# code from, say, C# - an 'a -> 'b -> 'c function there is indeed called the same way syntactically as an 'a * 'b -> 'c function - but this is more of an exception than a rule.
According to its signature Xunit.Assert.Equal() takes a single 2 values tuple parameter
I want to convert a string representations of few dozen enum types to enum values.
It's easy to convert string to concrete type:
Enum.Parse(typeof<FontStyle>,"Bold") |> unbox<FontStyle>
but for now i want to write function where type and string are parameters.
The best one i can write is:
> let s2e (_: 'a) s = Enum.Parse(typeof<'a>,s) |> unbox<'a>;;
val s2e : 'a -> string -> 'a
> s2e FontStyle.Regular "Bold";;
val it : FontStyle = Bold
Is there any option to write something like this but with type itself as first argument?
The function needs to take a single type parameter, which will be the type of the returned enum. However, you don't need to specify the type using a "fake" parameter that isn't used anywhere in the code - you can specify the actual type parameter when calling the function using the same notation as for example when calling defaultof<SomeType>.
The following modified function takes a single type parameter which occurs only in the return type (to avoid confusing SO code formatter, I replaced ' with ´ in the code):
> let parseEnum<´a> s = Enum.Parse(typeof<´a>,s) |> unbox<´a>;;
val parseEnum : string -> ´a
When calling the function, you need to specify the type explicitly:
> parseEnum<FontStyle> "Bold";;
val it : FontStyle = Bold
Your solution was pretty close to this - the only change is that you can specify the type parameter explicitly instead of providing a witness value to guide the type inference.
Tomas's answer is good. Additionally, note that you can apply F#'s enum constraints to prevent nonsensical type parameters from being used:
let parseEnum<'t,'u when 't:enum<'u>> s = System.Enum.Parse(typeof<'t>, s) :?> 't
(parseEnum "Class" : System.AttributeTargets)
Now calling (parseEnum "Test" : int) will fail at compile time since int is not an enum.
EDIT
Since we don't actually do anything with the underlying type 'u, we don't need the full power of F#'s enum constraints, as ssp points out in a comment. This is easier to use because it only has a single type parameter:
let parseEnum<'t when 't :> System.Enum and 't : struct> s =
System.Enum.Parse(typeof<'t>, s) :?> 't
Note that the struct constraint prevents using System.Enum itself as the type argument.