I'm trying to write an installer for my application that runs (mostly) a bunch of database migrations.
I have a very simple start here, I'm just trying to figure out what invokeMethod should equal.
let RunMigration (fsModule: System.Type) =
fsModule.GetProperties()
|> Array.filter (fun property -> property.Name = "run")
|> Array.map(fun property -> property |> invokeMethod ; "Ran successfully")
|> String.concat "<br/>"
let RunAllMigrations =
Assembly
.GetExecutingAssembly()
.GetTypes()
|> Array.filter FSharpType.IsModule
|> Array.filter (fun fsModule -> fsModule.Namespace = "DredgePos.Migrations")
|> Array.map RunMigration
|> String.concat "<br/><hr/>"
At the moment as you can see it is a fairly simple process of
Get all the types in the assembly
Getting all modules in the Migrations namespace.
Then if that module has a function named run, I want to invoke it.
How do I invoke it?
EDIT:
The run method will always be Unit -> Unit
let run () = ()
In your RunMigration method, you are getting a list of properties of the type - but a function declared as in your example should be compiled as a method. For example, if I have the following module in the current assembly:
module XX =
let run () = printfn "hello!"
It should be possible to invoke this if the RunMigration function first finds the run method and then calls the Invoke operation with no arguments (and null as the instance, since this is a static method):
let RunMigration (fsModule: System.Type) =
let mi = fsModule.GetMethod("run")
mi.Invoke(null,[||]) |> ignore
This returns obj value, which will be null (as the method returns unit) and so you can typically ignore the result.
I want to check if an argument passed to the constructor of a type is valid.
I check it and raise an ArgumentException if not valid.
I want to create a test for this behavior. I want to use Assert.throws or preferably FSUnit instead of a try/with block.
#package "FsUnit#3.4.1"
#package "nunit#3.11.0"
open System
open FSUnit
type configuration = {aaa:int}
type Client(conf:configuration) =
do
if conf.aaa < 3 then raise (ArgumentException("aaa must be at least 3"))
member this.do_something() =
()
// TEST
// 1. does not "compile"
Assert.Throws<ArgumentException>(fun () -> Client(configuration) |> ignore)
// 2. does not work
//Assert.Throws<ArgumentException>( fun () ->
// let a = Client(configuration);
// a
// |> ignore)
// 3. does not work
(fun() -> Client(configuration)) |> ignore |> should throw typeof<ArgumentException>
// 4. OK but... bleah!
try
Client(configuration) |> ignore
Assert.Fail()
with
| :? ArgumentException -> Assert.Pass() |> ignore
| _ -> Assert.Fail()
Your first approach works fine for me - I just had to define configuration which is not included in your question but, presumably, is defined somewhere in your actual file. The following compiles and behaves as expected for me:
let configuration = { aaa = 1 }
Assert.Throws<ArgumentException>(fun () -> Client(configuration) |> ignore)
Your second code snippet does not work because it has ignore in the wrong place - you are ignoring the entire function (which contains the code that you want to test) and then you are passing unit to the assertion. The ignore call needs to be inside of the function so that it ignores the result of calling the constructor. The following works for me:
(fun() -> Client(configuration) |> ignore) |> should throw typeof<ArgumentException>
I'm on a Mac running F# using .NET Core 2.0.
I have a function that looks like this:
let rec evaluate(x: string) =
match x with
// ... cases
| _ -> failwith "illogical"
I'd like to write an Expecto test that validates that the exception is thrown as expected, something along the lines of:
// doesn't compile
testCase "non-logic" <| fun _ ->
Expect.throws (evaluate "Kirkspeak") "illogical"
The error is
This expression was expected to have type
'unit -> unit' but here has type 'char'
unit -> unit makes me this is analogous to Assert.Fail, which is not what I want.
Being somewhat new to F# and Expecto, I'm having trouble locating a working example of asserting that an exception is thrown as expected. Does anyone have one?
Expect.throws has the signature (unit -> unit) -> string -> unit so the function you want to test must be (unit -> unit) or be wrapped inside a function that is (unit -> unit).
let rec evaluate (x: string) : char =
match x with
// ... cases
| _ -> failwith "illogical"
The compiler error is telling you that the function you passed to Expect.throws does not have the right signature yet.
[<Tests>]
let tests = testList "samples" [
test "non-logic" {
// (evaluate "Kirkspeak") is (string -> char)
// but expecto wants (unit -> unit)
Expect.throws (evaluate "Kirkspeak") "illogical"
}
]
[<EntryPoint>]
let main argv =
Tests.runTestsInAssembly defaultConfig argv
One way to make it work is to change
Expect.throws (evaluate "Kirkspeak") "illogical"
to
// you could instead do (fun () -> ...)
// but one use of _ as a parameter is for when you don't care about the argument
// the compiler will infer _ to be unit
Expect.throws (fun _ -> evaluate "Kirkspeak" |> ignore) "illogical"
Now expecto is happy!
This answer was the way I thought through it. It is usually helpful to follow the type signatures.
EDIT: I saw your error message saying This expression was expected to have type 'unit -> unit' but here has type 'char' so I updated my answer to match it.
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 trying to figure out how to properly test exceptions with FsUnit. Official documentation states, that to test for exceptions I have to right something like this:
(fun () -> failwith "BOOM!" |> ignore) |> should throw typeof<System.Exception>
But, if I don't mark my test method with [<ExpectedException>] attribute it will always fail. Sounds reasonable because if we want to test for exceptions we have to add such attribute in C# + NUnit.
But, as long as I've added this attribute it doesn't matter what kind of exception I'm trying to throw, it will be always handled.
Some snippets:
My LogicModule.fs
exception EmptyStringException of string
let getNumber str =
if str = "" then raise (EmptyStringException("Can not extract number from empty string"))
else int str
My LogicModuleTest.fs
[<Test>]
[<ExpectedException>]
let``check exception``()=
(getNumber "") |> should throw typeof<LogicModule.EmptyStringException>
Answer has been found. To test that exception was thrown I should wrap my function call in the next style:
(fun () -> getNumber "" |> ignore) |> should throw typeof<LogicModule.EmptyStringException>
because underneath #fsunit uses NUnit's Throws constraint
http://www.nunit.org/index.php?p=throwsConstraint&r=2.5 … which takes a delegate of void, raise returns 'a
If you want to test that a specific exception type is raised by some code, you can add the exception type to the [<ExpectedException>] attribute like so:
[<Test; ExpectedException(typeof<LogicModule.EmptyStringException>)>]
let``check exception`` () : unit =
(getNumber "")
|> ignore
More documentation is available on the NUnit site: http://www.nunit.org/index.php?p=exception&r=2.6.2