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 They seem outdated and can get you into unpleasant situation.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=','))
I am trying to run my code interactively in an fsx file. I have loaded all the dlls required, I then try to load the required files with #load but when I load the "Utlities.fs" file which depends on a function in the top file "HttpGetExchangeRate.fs" i get the error "Utilities.fs(88,42): error FS0039: The field, constructor or member 'getExchangeRates' is not defined"
Dose the 'getExchangeRates' not get defined when i load "HttpGetExchangeRate.fs"as in the image below or an I missing something?
#load "HttpGetExchangeRate.fs"
#load "Utilities.fs"
open System
open FsCheck
open NUnit.Framework
open HttpClient
InvoiceApp.Http.getExchangeRates "EUR" "USD"
InvoiceApp.Math.convertInvoicingCurrencyToEuro 200.00M "EUR"
Here is an image of the error message
If I understand your scenario correctly, this is due to a bug in how namespaces are handled in FSI. The workaround is to open the namespace you need before #loading the second file
#load "HttpGetExchangeRate.fs"
open InvoiceApp
#load "Utilities.fs"
That should get you unblocked for now, the bug has since been fixed (F# 4.0/VS 2015 will have the fix).
It sounds like you are running into the issue described in this question with implicit modules in fsi.
How to load external F# code and use it in fsi
I'm loading FSharp.Data in the interactive console. The library is loaded without any problem:
> #r "FSharp.Data.dll";;
--> Referenced 'C:\Users\pw\AppData\Local\Temp\FSharp.Data.dll' (file may be locked by F# Interactive process)
> open FSharp.Data;;
However, when I'm trying to initialize CsvProvider (defined in FSharp.Data) I get the error message saying the type is not defined:
> type Stocks = CsvProvider<"C:\Users\pw\Downloads\msft.csv">;;
type Stocks = CsvProvider<"C:\Users\pw\Downloads\msft.csv">;;
--------------^^^^^^^^^^^
stdin(62,15): error FS0039: The type 'CsvProvider' is not defined
I thought the problem may be with file and assemblies paths but now I'm using absolute paths and the error remains. On the other hand, I am able to use the CsvProvider when I'm creating a standard, not interactive, project. Any help to make it work in interactive session highly appreciated.
The warning about file being locked looks worrisome. Can you copy FSharp.Data somewhere and reference it using absolute path:
\#r #"C:\Poligon\packages\FSharp.Data.2.1.0\lib\net40\FSharp.Data.dll";;
Downgrade your FSharp.Core to 4.7 and FSharp.Data to 3.3.3. It should work after that.