I'm having a bit of trouble with the following FSharp/F# code:
module File1
let api a =
printf ("VALUE = %A") a
let router ops =
[|
api (ops (fun (list, _) -> list()))
api (ops (fun (_, get) -> get 1))
|]
let withContext ops handler =
let context = "CONTEXT"
handler (ops context)
let operations context =
printf ("CONTEXT = %s") context
let list () = [|1;2;3|]
let get id = "Test"
(list, get)
let setup() =
let ops = withContext operations
router ops
Results in the following error
Results in the following compation error
Error 1 Type mismatch. Expecting a
((unit -> int []) * (int -> int []) -> int []) -> 'a
but given a
((unit -> int []) * (int -> string) -> 'b) -> 'b
The type 'int []' does not match the type 'string'
I know the problem is that ops function has been bound to return a int[] but I want to be able to also return a string.
I think I'm missing a trick with some generic declarations but after hours of moving code around I can’t seem to work it out.
(I've simplified the code to highlight my problem)
The error is because ops needs to have a return type of its handler resolved at compilation, and you want to return different types base on some run-time logic.
It is basically an equivalent of:
let fun1 switch arg2 arg3 =
if switch then
arg2
else
arg3
and you want to run it this way:
fun1 true 1 "string"
Of course, arg2 and arg3 need to have the same type, so it won't work
What you can do is to run "api" function on a handler result, before returning it (so it will always the same type - unit).
let router ops =
[|
ops (fun (list, _) -> api <| list()))
ops (fun (_, get) -> api <| get 1))
|]
Alternatively, you could return objects of discriminated union type (then you will need some more logic in api function).
(Technically, you could also return obj).
Bonus
You don't need the array of units to be returned in a router function, returning one unit is just fine:
let router ops =
ops (fun (list, _) -> api <| list()))
ops (fun (_, get) -> api <| get 1))
In this way, setup function will also return unit and you will be able to run it without a need to run ignore on the result to get rid of This expression should have type 'unit', but has type 'unit[]' warning.
Your code is hard for me to understand, but I think the basic issue is that you want withContext to have a "rank 2" type (so that the universal quantification of the type variable 'b can happen after the application of the first argument). In F#, this can be accomplished by creating a new type with a generic method and using that:
let api a =
printf ("VALUE = %A") a
type Handler<'a> = abstract Handle<'b> : f:('a->'b) -> 'b
let router (ops:Handler<_>) =
[|
api (ops.Handle (fun (list, _) -> list()))
api (ops.Handle (fun (_, get) -> get 1))
|]
let withContext ops =
let context = "CONTEXT"
{ new Handler<_> with member __.Handle f = f (ops context) }
let operations context =
printf ("CONTEXT = %s") context
let list () = [|1;2;3|]
let get id = "Test"
(list, get)
let setup() =
let ops = withContext operations
router ops
Related
Previously had a very compact and comprehensive answer for my question.
I had it working for my custom type but now due to some reason I had to change it to string type which is now causing type mismatch errors.
module AsyncResult =
let bind (binder : 'a -> Async<Result<'b, 'c>>) (asyncFun : Async<Result<'a, 'c>>) : Async<Result<'b, 'c>> =
async {
let! result = asyncFun
match result with
| Error e -> return Error e
| Ok x -> return! binder x
}
let compose (f : 'a -> Async<Result<'b, 'e>>) (g : 'b -> Async<Result<'c, 'e>>) = fun x -> bind g (f x)
let (>>=) a f = bind f a
let (>=>) f g = compose f g
Railway Oriented functions
let create (json: string) : Async<Result<string, Error>> =
let url = "http://api.example.com"
let request = WebRequest.CreateHttp(Uri url)
request.Method <- "GET"
async {
try
// http call
return Ok "result"
with :? WebException as e ->
return Error {Code = 500; Message = "Internal Server Error"}
}
test
type mismatch error for the AsyncResult.bind line
let chain = create
>> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
match chain "initial data" |> Async.RunSynchronously with
| Ok data -> Assert.IsTrue(true)
| Error error -> Assert.IsTrue(false)
Error details:
EntityTests.fs(101, 25): [FS0001] Type mismatch. Expecting a '(string -> string -> Async<Result<string,Error>>) -> 'a' but given a 'Async<Result<'b,'c>> -> Async<Result<'d,'c>>' The type 'string -> string -> Async<Result<string,Error>>' does not match the type 'Async<Result<'a,'b>>'.
EntityTests.fs(101, 25): [FS0001] Type mismatch. Expecting a '(string -> string -> Async<Result<string,Error>>) -> 'a' but given a 'Async<Result<string,'b>> -> Async<Result<string,'b>>' The type 'string -> string -> Async<Result<string,Error>>' does not match the type 'Async<Result<string,'a>>'.
Edit
Curried or partial application
In context of above example, is it the problem with curried functions? for instance if create function has this signature.
let create (token: string) (json: string) : Async<Result<string, Error>> =
and then later build chain with curried function
let chain = create "token" >> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
Edit 2
Is there a problem with following case?
signature
let create (token: Token) (entityName: string) (entityType: string) (publicationId: string) : Async<Result<string, Error>> =
test
let chain = create token >> AsyncResult.bind ( fun (result: string) -> async {return Ok "more results"} )
match chain "test" "article" "pubid" |> Async.RunSynchronously with
Update: At the front of the answer, even, since your edit 2 changes everything.
In your edit 2, you have finally revealed your actual code, and your problem is very simple: you're misunderstanding how the types work in a curried F# function.
When your create function looked like let create (json: string) = ..., it was a function of one parameter. It took a string, and returned a result type (in this case, Async<Result<string, Error>>). So the function signature was string -> Async<Result<string, Error>>.
But the create function you've just shown us is a different type entirely. It takes four parameters (one Token and three strings), not one. That means its signature is:
Token -> string -> string -> string -> Async<Result<string, Error>>
Remember how currying works: any function of multiple parameters can be thought of as a series of functions of one parameter, which return the "next" function in that chain. E.g., let add3 a b c = a + b + c is of type int -> int -> int -> int; this means that add3 1 returns a function that's equivalent to let add2 b c = 1 + b + c. And so on.
Now, keeping currying in mind, look at your function type. When you pass a single Token value to it as you do in your example (where it's called as create token, you get a function of type:
string -> string -> string -> Async<Result<string, Error>>
This is a function that takes a string, which returns another function that takes a string, which returns a third function which takes a string and returns an Async<Result<whatever>>. Now compare that to the type of the binder parameter in your bind function:
(binder : 'a -> Async<Result<'b, 'c>>)
Here, 'a is string, so is 'b, and 'c is Error. So when the generic bind function is applied to your specific case, it's looking for a function of type string -> Async<Result<'b, 'c>>. But you're giving it a function of type string -> string -> string -> Async<Result<string, Error>>. Those two function types are not the same!
That's the fundamental cause of your type error. You're trying to apply a function that returns a function that returns function that returns a result of type X to a design pattern (the bind design pattern) that expects a function that returns a result of type X. What you need is the design pattern called apply. I have to leave quite soon so I don't have time to write you an explanation of how to use apply, but fortunately Scott Wlaschin has already written a good one. It covers a lot, not just "apply", but you'll find the details about apply in there as well. And that's the cause of your problem: you used bind when you needed to use apply.
Original answer follows:
I don't yet know for a fact what's causing your problem, but I have a suspicion. But first, I want to comment that the parameter names for your AsyncResult.bind are wrong. Here's what you wrote:
let bind (binder : 'a -> Async<Result<'b, 'c>>)
(asyncFun : Async<Result<'a, 'c>>) : Async<Result<'b, 'c>> =
(I moved the second parameter in line with the first parameter so it wouldn't scroll on Stack Overflow's smallish column size, but that would compile correctly if the types were right: since the two parameters are lined up vertically, F# would know that they are both belonging to the same "parent", in this case a function.)
Look at your second parameter. You've named it asyncFun, but there's no arrow in its type description. That's not a function, it's a value. A function would look like something -> somethingElse. You should name it something like asyncValue, not asyncFun. By naming it asyncFun, you're setting yourself up for confusion later.
Now for the answer to the question you asked. I think your problem is this line, where you've fallen afoul of the F# "offside rule":
let chain = create
>> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
Note the position of the >> operator, which is to the left of its first operand. Yes, the F# syntax appears to allow that in most situations, but I suspect that if you simply change that function definition to the following, your code will work:
let chain =
create
>> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
Or, better yet because it's good style to make the |> (and >>) operators line up with their first operand:
let chain =
create
>> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
If you look carefully at the rules that Scott Wlaschin lays out in https://fsharpforfunandprofit.com/posts/fsharp-syntax/, you'll note that his examples where he shows exceptions to the "offside rule", he writes them like this:
let f g h = g // defines a new line at col 15
>> h // ">>" allowed to be outside the line
Note how the >> character is still to the right of the = in the function definition. I don't know exactly what the F# spec says about the combination of function definitions and the offside rule (Scott Wlaschin is great, but he's not the spec so he could be wrong, and I don't have time to look up the spec right now), but I've seen it do funny things that I didn't quite expect when I wrote functions with part of the function definition on the same line as the function, and the rest on the next line.
E.g., I once wrote something like this, which didn't work:
let f a = if a = 0 then
printfn "Zero"
else
printfn "Non-zero"
But then I changed it to this, which did work:
let f a =
if a = 0 then
printfn "Zero"
else
printfn "Non-zero"
I notice that in Snapshot's answer, he made your chain function be defined on a single line, and that worked for him. So I suspect that that's your problem.
Rule of thumb: If your function has anything after the = on the same line, make the function all on one line. If your function is going to be two lines, put nothing after the =. E.g.:
let f a b = a + b // This is fine
let g c d =
c * d // This is also fine
let h x y = x
+ y // This is asking for trouble
I would suspect that the error stems from a minor change in indentation since adding a single space to an FSharp program changes its meaning, the FSharp compiler than quickly reports phantom errors because it interprets the input differently. I just pasted it in and added bogus classes and removed some spaces and now it is working just fine.
module AsyncResult =
[<StructuralEquality; StructuralComparison>]
type Result<'T,'TError> =
| Ok of ResultValue:'T
| Error of ErrorValue:'TError
let bind (binder : 'a -> Async<Result<'b, 'c>>) (asyncFun : Async<Result<'a, 'c>>) : Async<Result<'b, 'c>> =
async {
let! result = asyncFun
match result with
| Error e -> return Error e
| Ok x -> return! binder x
}
let compose (f : 'a -> Async<Result<'b, 'e>>) (g : 'b -> Async<Result<'c, 'e>>) = fun x -> bind g (f x)
let (>>=) a f = bind f a
let (>=>) f g = compose f g
open AsyncResult
open System.Net
type Assert =
static member IsTrue (conditional:bool) = System.Diagnostics.Debug.Assert(conditional)
type Error = {Code:int; Message:string}
[<EntryPoint>]
let main args =
let create (json: string) : Async<Result<string, Error>> =
let url = "http://api.example.com"
let request = WebRequest.CreateHttp(Uri url)
request.Method <- "GET"
async {
try
// http call
return Ok "result"
with :? WebException as e ->
return Error {Code = 500; Message = "Internal Server Error"}
}
let chain = create >> AsyncResult.bind (fun (result: string) -> (async {return Ok "more results"}))
match chain "initial data" |> Async.RunSynchronously with
| Ok data -> Assert.IsTrue(true)
| Error error -> Assert.IsTrue(false)
0
I'm messing with my new statically parameterized type provider that provides a type with statically parameterized static methods. I haven't found documentation about this not being allowed. I'm getting some strange type provider behavior:
This type-provider-consuming code runs correctly but the intellisense gives crappy info. Members just keep getting added but never removed. The OpDef method shouldn't be available without the type parameters but intellisense kinda shows that as an option but still gives an error about a missing invoker if I actually refer to it. The required arguments to the provided OpDef method are not shown - always shows OpDef() -> unit instead of OpDef(suffix : string * id : string) -> unit that I currently expect to see (from this draft of the type provider). But as I've given all the arguments it really needs then it stops complaining.
Am I doing something that isn't supported or correct? Or, doubtfully, is there a bug in the f# stuff (and can you isolate where the bug is apparent)?
Here is the full implementation which uses the starter-pack files.
namespace OfflineSql
open ProviderImplementation.ProvidedTypes
open Microsoft.FSharp.Core.CompilerServices
open System.Text.RegularExpressions
#nowarn "0025"
[<TypeProvider>]
type OfflineSqlProvider (config : TypeProviderConfig) as this =
inherit TypeProviderForNamespaces ()
let ns = "OfflineSql"
let asm = System.Reflection.Assembly.GetExecutingAssembly()
let buildDomainProvider nm schema invariants =
let parameterizedType = ProvidedTypeDefinition(asm, ns, nm, None)
let m = ProvidedMethod("OpDef", list.Empty, typeof<unit>, IsStaticMethod = true)
m.DefineStaticParameters(
[
ProvidedStaticParameter("script", typeof<string>)
],
fun nm [| :? string as operation |] ->
let results =
Regex.Matches(operation, "#([a-zA-Z_][a-zA-Z0-9_]*)") |> Seq.cast
|> Seq.map (fun (regmatch: Match) -> regmatch.Groups.[1].Captures.[0].ToString())
let ps = [ for a in results -> ProvidedParameter(a, typeof<string>) ] |> List.distinct
let opDef = ProvidedMethod(nm, ps, typeof<unit>, IsStaticMethod = true, InvokeCode = (fun _ -> <## () ##>))
opDef.AddXmlDoc("Constructs a guarded method for the operation's script")
parameterizedType.AddMember(opDef)
opDef)
let schemaProp =
ProvidedProperty("Schema", typeof<string>, IsStatic = true,
GetterCode = (fun _ -> <## schema ##>))
let invariantsProp =
ProvidedProperty("Invariants", typeof<string>, IsStatic = true,
GetterCode = (fun _ -> <## invariants ##>))
parameterizedType.AddMember(m)
parameterizedType.AddMember(schemaProp)
parameterizedType.AddMember(invariantsProp)
parameterizedType
do
let root = ProvidedTypeDefinition(asm, ns, "Domain", None)
root.DefineStaticParameters(
[
ProvidedStaticParameter("schema", typeof<string>)
ProvidedStaticParameter("invariants", typeof<string>, "")
],
fun nm [| :? string as schema ; :? string as invariants |] ->
buildDomainProvider nm schema invariants)
this.AddNamespace(ns, [ root ])
[<assembly:TypeProviderAssembly>]
do ()
This is directly related to known bugs with a fix:
https://github.com/Microsoft/visualfsharp/issues/642
https://github.com/Microsoft/visualfsharp/issues/640
https://github.com/Microsoft/visualfsharp/pull/705
Thanks to #gauthier on functionalprogramming.slack.com/fsharp for this find.
I'm trying to create some kind of interface, but i cannot find how to use custom attributes in F# as MSDN only shows usage of CLR attributes. This is what i want to achieve:
open System
type Command (name : string) =
inherit Attribute()
member this.Name = name
[<Command("something")>]
let doSomething () =
Console.Write("I'm doing something")
[<Command("somethingElse")>]
let doSomethingElse () =
Console.Write("I'm doing something else")
[<EntryPoint>]
let main args =
let command = Console.ReadLine()
// find function where Command.Name = command and call it
Console.Read()
0
To extend on your answer, a more generic approach would be to get all the types and then filter the functions that have the attribute you're looking for (as your approach would break down once your application grows and no longer has everything "packed" into the Program class):
let getCommands () =
let types = Assembly.GetExecutingAssembly().GetTypes()
let commands =
types
|> Array.collect (fun typ -> typ.GetMethods())
|> Array.choose (fun mi ->
mi.CustomAttributes
|> Seq.tryFind (fun attr -> attr.AttributeType = typeof<Command>)
|> Option.map (fun attr -> attr, mi))
let commandsMap =
commands
|> Seq.map (fun (attr, mi) ->
let name =
let arg = attr.ConstructorArguments.[0]
unbox<string> arg.Value
name, mi)
|> Map.ofSeq
commandsMap
This gets all the functions from all the types in the executing assembly, then filters out everything that doesn't have command attribute. Then it builds a map where the key is the attribute argument and the value is the MethodInfo of the function.
Ok, found it.
Reflection.Assembly.GetExecutingAssembly().GetType("Program").GetMethods()
Program typename is not viable in code so it cannot be used in typeof<Program>, but this type exists and can be taken from assembly.
I'm quite sure that I run into some kind of limitation, but I do not understand it:
type IRunner =
abstract member Run : (string -> 'a) -> 'a
type T() =
let run4 doFun = doFun "4"
let run5 doFun = doFun "5"
let parseInt s = System.Int32.Parse(s)
let parseFloat s = System.Double.Parse(s)
let doSomething () =
let i = parseInt |> run4
let f = parseFloat |> run4
f |> ignore
// Make it more generic ->
//let doSomething2 (runner:(string->'a)->'b) =
let doSomething2 runner =
// Error on the following lines with both declarations
let i = parseInt |> runner
let f = parseFloat |> runner
f |> ignore
// Want to do something like
let test () =
doSomething2 run4
doSomething2 run5
// Workaround
let workaround (runner:IRunner) =
let run f = runner.Run f
let i = parseInt |> run
let f = parseFloat |> run
f |> ignore
Can somebody bring some light over this? I did not find any related question, sorry if i duplicated something.
The problem is, if doSomething2 has type ((string->'a) -> 'b) -> unit, then 'a and 'b are fixed during each invocation of doSomething2, which isn't what you want - in your case 'a needs to treated as both int and float during a single invocation of doSomething2.
It seems like what you really want is more like: doSomething2 : (forall 'a. (string -> 'a) -> 'a) -> unit, but that kind of direct universal quantification doesn't exist in F#. As you've discovered, the way to work around this is to use a type with a generic method.
And even if F# did support forall types, as I mentioned in a comment inference still wouldn't be possible. Consider your doSomething2 function - we know that runner needs to be able to take an input of type string -> int to some output type and an input of type string -> float to some (possibly different) output type. Here are several different signatures for doSomething2 that all meet this requirement:
forall 'a. 'a -> 'a
forall 'a. (string -> 'a) -> 'a
forall 'a. 'a -> unit
Note that none of these types is more general than the others, they are all incompatible. In the first case, we could pass id to the function, in the second case, we could pass run4 to it, and in the third case, we could pass ignore to it (but none of those functions is compatible with the other possible signatures!).
I continue to work on a printer for F# quoted expressions, it doesn't have to be perfect, but I'd like to see what is possible. The active patterns in Microsoft.FSharp.Quotations.Patterns and Microsoft.FSharp.Quotations.DerivedPatterns used for decomposing quoted expressions will typically provide MemberInfo instances when appropriate, these can be used to obtain the name of a property, function, etc. and their "declaring" type, such as a module or static class. The problem is, I only know how to obtain the CompiledName from these instances but I'd like the F# name. For example,
> <# List.mapi (fun i j -> i+j) [1;2;3] #> |> (function Call(_,mi,_) -> mi.DeclaringType.Name, mi.Name);;
val it : string * string = ("ListModule", "MapIndexed")
How can this match be rewritten to return ("List", "mapi")? Is it possible?
FYI, here is my final polished solution from Stringer Bell and pblasucci's help:
let moduleSourceName (declaringType:Type) =
FSharpEntity.FromType(declaringType).DisplayName
let methodSourceName (mi:MemberInfo) =
mi.GetCustomAttributes(true)
|> Array.tryPick
(function
| :? CompilationSourceNameAttribute as csna -> Some(csna)
| _ -> None)
|> (function | Some(csna) -> csna.SourceName | None -> mi.Name)
//usage:
let sourceNames =
<# List.mapi (fun i j -> i+j) [1;2;3] #>
|> (function Call(_,mi,_) -> mi.DeclaringType |> moduleSourceName, mi |> methodSourceName);
You can use F# powerpack for that purpose:
open Microsoft.FSharp.Metadata
...
| Call(_, mi, _) ->
let ty = Microsoft.FSharp.Metadata.FSharpEntity.FromType(mi.DeclaringType)
let name = ty.DisplayName // name is List
However, I don't think if it's possible to retrieve function name with powerpack.
Edit:
As hinted by pblasucci, you can use CompilationSourceName attribute for retrieving source name:
let infos = mi.DeclaringType.GetMember(mi.Name)
let att = infos.[0].GetCustomAttributes(true)
let fName =
(att.[1] :?> CompilationSourceNameAttribute).SourceName // fName is mapi