How is it possible that the F# compiler rejects the type string when it's expecting a System.String?
#I #"..\..\packages"
#r #"FSharp.Data\lib\net40\FSharp.Data.dll"
open FSharp.Data
let [<Literal>] csvFile = #"..\..\data\FootballResults.csv"
type Football = CsvProvider< csvFile >
let data = Football.GetSample().Rows |> Seq.toArray
After running this simple code, I get the error message:
Script.fsx(5,30): error FS0001: This expression was expected to have type 'System.String' but here has type 'string'
I thought string was simply an alias for System.String?
Edit
The csv file is quite big, just the first rows are important:
Date,Home Team,Away Team,Full Time Home Goals,Full Time Away Goals,Full Time Result,Half Time Home Goals,Half Time Away Goals,Half Time Result,Home Shots,Away Shots,Home Shots on Target,Away Shots on Target,Home Fouls,Away Fouls,Home Cards,Away Cards,Home Yellow Cards,Away Yellow Cards,Home Red Cards,Away Red Cards
08/18/2012,Arsenal,Sunderland,0,0,D,0,0,D,14,3,4,2,12,8,7,0,0,0,0,0
08/18/2012,Fulham,Norwich,5,0,H,2,0,H,11,4,9,2,12,11,6,3,0,0,0,0
08/18/2012,Newcastle,Tottenham,2,1,H,0,0,D,6,12,4,6,12,8,3,5,2,2,0,0
The full file can be downloaded here under the folder data
FSharp.Data version
FSharp.Data 2.3.2
I found the problem, it was in the paket script provided in the book Get Programming with F#.
The script was pointing to an old version of FSharp.Data (I thought paket was this amazing package manager that always get you the best version for your project while in fact, it does not, you always fight and struggle with wrong dependencies).
At the end I managed to fix the problem this way:
Solution 1
Simply start a new project using the nuget command Install-Package FSharp.Data -Version 4.2.7 and then the reference to the package #r "nuget: FSharp.Data"
Solution 2
Update the paket.lock provided by the book with the correct version FSharp.Data (4.2.7). The code posted in my question run after that.
Advice for F# learner
Whoever is trying to learn F# with this book, it's a good book but be very careful with their package reference. They seem outdated and can get you into unpleasant situation.
I am writing a service using .net Core with Visual Studio 2017. I want to use JSON for the configuration, so I defined my type like this:
type ServiceConfig = JsonProvider<"exampleConfig.json", EmbeddedResource="MyService, exampleConfig.json", SampleIsList = true>
In my program, I load it like this:
let conf = ServiceConfig.Load "config.json"
When I run it, it works fine, but in VS 2017 I get a red squiggly line and the error log says:
FS3033 The type provider 'ProviderImplementation.JsonProvider'
reported an error in the context of provided type
'FSharp.Data.JsonProvider,Sample="exampleConfig.json",SampleIsList="True",EmbeddedResource="MyService, exampleConfig.json"', member 'Load'. The error: Method
'FSharp.Data.Runtime.BaseTypes.IJsonDocument
Create(System.IO.TextReader, System.String)' not found in type
''. This method may be missing in the types available in the
target assemblies.
How do I get rid of this?
I'm not sure that this is the problem, but it might help to give an absolute path of the sample file. Resolving a relative path is a constant source of issues in type providers. You can do something like:
[<Literal>]
let sample = __SOURCE_DIRECTORY__ + "/exampleConfig.json"
type ServiceConfig =
JsonProvider< sample, EmbeddedResource="MyService, e
xampleConfig.json", SampleIsList = true >
By chance I found the answer myself. It must have been some Visual Studio index having gone wonky. I had another problem with another project in my solution so I just did an "emergency commit" to git, cleaned my local working copy using git clean -fdx and then reloaded the solution, rebuilt everything and the errors went away.
I am new to F#/.NET and I am trying to run the F# example provided in the accepted answer of How to translate the intro ML.Net demo to F#? with the ML.NET library, using F# on Visual Studio, using Microsoft.ML (0.2.0).
When building it I get the error error FS0039: The type 'TextLoader' is not defined.
To avoid this, I added the line
open Microsoft.ML.Data
to the source.
Then, however, the line
pipeline.Add(new TextLoader<IrisData>(dataPath,separator = ","))
triggers:
error FS0033: The non-generic type 'Microsoft.ML.Data.TextLoader' does not expect any type arguments, but here is given 1 type argument(s)
Changing to:
pipeline.Add(new TextLoader(dataPath,separator = ","))
yields:
error FS0495: The object constructor 'TextLoader' has no argument or settable return property 'separator'. The required signature is TextLoader(filePath: string) : TextLoader.
Changing to:
pipeline.Add(new TextLoader(dataPath))
makes the build successful, but the code fails when running with
ArgumentOutOfRangeException: Column #1 not found in the dataset (it only has 1 columns), I assume because the comma separator is not correctly picked up (incidentally, you can find and inspect the iris dataset at https://archive.ics.uci.edu/ml/machine-learning-databases/iris/iris.data).
Also
pipeline.Add(new TextLoader(dataPath).CreateFrom<IrisData>(separator: ','))
won't work.
I understand that there have been changes in TextLoader recently (see e.g. https://github.com/dotnet/machinelearning/issues/332), can somebody point me to what I am doing wrong?
F# just has a bit of a different syntax that can take some getting used to. It doesn't use the new keyword to instantiate a new class and to use named parameters it uses the = instead of : that you would in C#.
So for this line in C#:
pipeline.Add(new TextLoader(dataPath).CreateFrom<IrisData>(separator: ','))
It would be this in F#:
pipeline.Add(TextLoader(dataPath).CreateFrom<IrisData>(separator=','))
While running one piece of code using Alea.cuBase I am getting a type initialization exception
let worker = Engine.workers.DefaultWorker
System.TypeInitializationException was unhandled
Message: The type initializer for 'Worker' threw an exception.
I am using
visual studio 2012
.Net frame work 4.0
F# 3.1
Another piece of code I tried and got the same error at,
useprogram = template |> Compiler.loadWorker.Default
I started with trial version of Alea.cuBase 1.3.914.
Guide me how to fix this..
Problem got resolved.
My pc was having graphics card quadro fx 1800.
Because Alea cuBase support fermi or higher architecture.
When I tried with k4000 I could run the same program.
I am trying to use the FSharp.Data XmlProvider.
According to the samples you can access nested types and create a function that will receive a parameter of one of those types.
https://github.com/fsharp/FSharp.Data/blob/master/samples/library/XmlProvider.fsx (line 177)
However when I try to do the following:
type businessesT = XmlProvider<"Businesses.xml">
type businessT = businessesT.DomainTypes.Business
let testfunc (b:businessesT.DomainTypes.Business) =
b
It seems to work until I actually compiles and then I get
error FS0039: The type 'Business' is not defined
UPDATE:
The problem can be reproduced with the sample from FSharp.Data (XmlProvider.fsx)
adding a type alias after line 205
type Rss = XmlProvider<"http://tomasp.net/blog/rss.aspx">
type test = Rss.DomainTypes.Channel
What is strange is that the sample with the printDiv function is working...
This is most likely caused by some problem with loading the type provider - if the compiler fails to load the type provider, then it cannot run it and so none of the provided types like Business would be defined.
Are you using #r in a script file, or are you referencing the type provider through "Add References" in a project? If you're using #r, check if there is any error message on that line. In case of project, check other error messages output by the compiler.
I already listed some common reasons why type provider fails to load in another answer.
This was probably the same problem as "type provider" not recognized when building project, which has been fixed in FSharp.Data 1.1.10