Command Line Tools: Loading a Script Interactively

Last post, I loaded a dll in the interactive shell (REPL) and called a method. This time, I want to explore a similar option: loading code from an external script. I've got a script adapted from the examples on http://www.tryfsharp.org/ and I'll use that for my example.

The questions I'd like to answer for myself:

• How do I load an external F# script and call methods on it in the REPL?

I've got this saved in fsharp.fs:

// This code is saved in fsharp.fs
// "Date,Open,High,Low,Close,Volume,Adj Close"
let stockData =
    [ 
      "2012-03-30,32.40,32.41,32.04,32.26,31749400,32.26";
      "2012-03-29,32.06,32.19,31.81,32.12,37038500,32.12";
      "2012-03-28,32.52,32.70,32.04,32.19,41344800,32.19";
      "2012-03-27,32.65,32.70,32.40,32.52,36274900,32.52";
      "2012-03-26,32.19,32.61,32.15,32.59,36758300,32.59";
      "2012-03-23,32.10,32.11,31.72,32.01,35912200,32.01";
      "2012-03-22,31.81,32.09,31.79,32.00,31749500,32.00";
      "2012-03-21,31.96,32.15,31.82,31.91,37928600,31.91";
      "2012-03-20,32.10,32.15,31.74,31.99,41566800,31.99";
      "2012-03-19,32.54,32.61,32.15,32.20,44789200,32.20";
      "2012-03-16,32.91,32.95,32.50,32.60,65626400,32.60";
      "2012-03-15,32.79,32.94,32.58,32.85,49068300,32.85";
      "2012-03-14,32.53,32.88,32.49,32.77,41986900,32.77";
      "2012-03-13,32.24,32.69,32.15,32.67,48951700,32.67";
      "2012-03-12,31.97,32.20,31.82,32.04,34073600,32.04";
      "2012-03-09,32.10,32.16,31.92,31.99,34628400,31.99";
      "2012-03-08,32.04,32.21,31.90,32.01,36747400,32.01";
      "2012-03-07,31.67,31.92,31.53,31.84,34340400,31.84";
      "2012-03-06,31.54,31.98,31.49,31.56,51932900,31.56";
      "2012-03-05,32.01,32.05,31.62,31.80,45240000,31.80";
      "2012-03-02,32.31,32.44,32.00,32.08,47314200,32.08";
      "2012-03-01,31.93,32.39,31.85,32.29,77344100,32.29";
      "2012-02-29,31.89,32.00,31.61,31.74,59323600,31.74"; ]

let splitCommas (x:string) =
    x.Split([|','|])

let processStockData sData =
    sData
    |> List.map splitCommas
    |> List.maxBy (fun x -> abs(float x.[1] - float x.[4]))
    |> (fun x -> x.[0])

Now, in theory, I should be able to load this script and call either of the functions that have been defined here. I haven't done this before, but let's give this a try. The help menu says this:

#load "file.fs" ...;; Load the given file(s) as if compiled and referenced

Okay, so in theory I should be able to do this:

> #load "fsharp.fs";; 

[Loading /Users/Matthew/Desktop/sharp.fs]

/Users/Matthew/Desktop/sharp.fs(2,1): error FS0222: Files in libraries or multiple-file applications must begin with a namespace or module declaration, e.g. 'namespace SomeNamespace.SubNamespace' or 'module SomeNamespace.SomeModule'. Only the last source file of an application may omit such a declaration.

Oops. Well, that didn't quite work. The internets tell me slightly more than this verbose error message. I'm going to add a module statement at the top of the fsharp.fs file:

module Fsharp

And now let's try it again:

> #load "fsharp.fs";;

[Loading /Users/Matthew/Desktop/sharp.fs]

namespace FSI_0006

  val stockData : string list

  val splitCommas : x:string -> string []

  val processStockData : sData:string list -> string

Looks like that worked. It's worth noting that at this point I really have only the vaguest notion of what either module or namespace mean in the context of F#. Of course, my experience with other languages give me some pre-conceived notions but in the case of F# I'm just taking this on faith. I think this is an important concept in learning new technology: don't let not knowing get you stuck. The distinction between these ideas will become clear in time. For now, I'm focused on getting the REPL to include an external F# script...

Now the moment of truth:

> processStockData stockData;;

  processStockData stockData;;

  ^^^^^^^^^^^^^^^^

/Users/Matthew/Desktop/stdin(13,1): error FS0039: The value or constructor 'processStockData' is not defined

Oof, not defined. Well, it said when I loaded the script that processStockData was defined, but I'm guessing it's nested inside of the namespace listed above. Okay, so I've got to figure that out. I've got a hunch:

/Users/Matthew/Desktop/stdin(14,1): error FS0039: The namespace or module 'Fsharp' is not defined

> Fsharp.processStockData stockData;;

  Fsharp.processStockData stockData;;

  -----------------------^^^^^^^^^

/Users/Matthew/Desktop/stdin(15,24): error FS0039: The value or constructor 'stockData' is not defined

Hey, look at that: Fsharp is the name I gave the module, and I used that to prefix the method and now it's complaining about missing stockData. So it stands to reason that I should be able to prefix that as well:

> Fsharp.processStockData Fsharp.stockData;;

val it : string = "2012-03-13"

Boom! You can work through the original tutorial yourself but I can tell you that "2012-03-13" is the expected output.

So this session answered a few questions but raised a few others:

• What's the difference between a module and a namespace?
• How do you import a namespace into the current namespace?
• What are the best practices for handling modules vs. namespaces? Naming conventions?

GOTO

Referencing a DLL Interactively