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 They seem outdated and can get you into unpleasant situation.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 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 using Seq.tryHead but I am getting an error
let maybeTagDatabaseModel = Seq.tryHead tagSeq
error
error FS39: The value, constructor, namespace or type 'tryHead' is not defined
Does anyone have suggestions on how to fix? Thanks
For future visitors
The problem seemed to be this dependency "FSharp.Interop.Dynamic": "3.0.0" in the project.json file.
Two of my projects were on version less than 4. Upgraded those and they work now.
I'm trying to translate some code from C# to F#, specifically the code to create a shortcut, from here: http://vbaccelerator.com/home/NET/Code/Libraries/Shell_Projects/Creating_and_Modifying_Shortcuts/ShellLink_Code_zip_ShellLink/ShellLink_cs.asp
The code in C# reads:
[GuidAttribute("00021401-0000-0000-C000-000000000046")]
[ClassInterfaceAttribute(ClassInterfaceType.None)]
[ComImportAttribute()]
private class CShellLink{}
Which I translated to F# as:
[<GuidAttribute("00021401-0000-0000-C000-000000000046")>]
[<ClassInterfaceAttribute(ClassInterfaceType.None)>]
[<ComImportAttribute()>]
type CShellLink() = class end
Unfortunately, when I switch to the F# implementation, I get a runtime error of: "Method with non-zero RVA in an Import". This seems to be the same error as reported here: http://social.msdn.microsoft.com/Forums/en-US/fsharpgeneral/thread/dada2004-5218-4089-8918-eed2464bbbcd
Is there any workaround? I'm trying to port the application to only use F#, so if this can't be written in F# that project is going to have to be rethought.
this looks like a limitation in F# compiler: it cannot define existing COM classes using ComImportAttribute, it only works for interfaces. Can you use this as workaround?
let shellLink =
let ty = System.Type.GetTypeFromCLSID (System.Guid "00021401-0000-0000-C000-000000000046")
Activator.CreateInstance ty
I am working ti old F# code from Expert F#. However, the example doesn't build anymore.
The following two calls don't seem to exist:
semaphore.AsyncWaitOne(?millisecondsTimeout=timeout)
and
reader.ReadToEndAsync()
Does anyone know what these have been replaced with or if I am just missing a reference?
It's now called Async.AwaitWaitHandle.
AsyncReadToEnd is in the F# PowerPack.
By now, FSharp PowerPack project has been broken up into smaller modules for any further development.
Specifically, the AsyncStreamReader class and the extension methods for the reading from a StreamReader, WebClient, etc. the new project is FSharpx.Async.
1) AsyncWaitOne is now called Async.AwaitWaitHandle.
2) AsyncReadToEnd() extension method does not exists anymore in the FSharp.PowerPack.
It has been replaced with the AsyncStreamReader dedicated type that contains proper asynchronous implementation of stream reading (like ReadToEnd, ReadLine, etc.)
It can be used like that:
async {
use asyncReader = new AsyncStreamReader(stream)
return! asyncReader.ReadToEnd() }
Note: Once you have installed FSharp.PowerPack, the AsyncStreamReader type will be 'injected' in the Microsoft.FSharp.Control namespace