F# 'modular' scripting

Question

What is the recommended way to load+reload fsx files? Just experimenting... yes yes right language right job ect ect..

I love how the following can be done in FSI:

#load "script.fsx";
open Script

> let p = script.x 1 
  Error: This expression was expected to have type string but here has int...
(* edit script.fsx x to make it int -> int  *)
>
> #load "script.fsx"
> let p = script.x 1
val it : int = 2

But how do we do this for an application that we are running via fsi blah.fsx? Maybe something that is sitting in a while loop. It seems #load and #use must not be inside let or module.. i.e. you cannot use #load like let reload script = #load script, wonder why?

My original method was to have .fs files and recompile + relaunch each time I wanted to add/fix something. This method feels primitive.

Second method was to attempt to use the #load directive inside of a module, which turns out to not work (kind of makes sense in terms of scoping)...

module test1 =
  #load @"C:\users\pc\Desktop\test.fsx"

  open Test

module test2 = 
  ...

Another way would be to create a new process for every module by loading fsi module.fsx with process diagnostics, but this seems horrible, inefficient and ugh.

I have a feeling deep in my heart that this will not be trivial inside .NET, but I would like to pose the question anyway, FSI does it... I wonder if I can leverage the FSI API or something (or at the least to copy their code)?

TL;DR I read the following about erlang and want it for myself in F#. Erlang: Is there a way to reload changed modules into an already running node with rebar?

"...any time a module in your program changes on disk, the reloader will replace the running copy automatically."

There might be a solution - using fsiDynamicCompiler.EvalLoadFiles - this is what #load calls. For reference see github.com/fsharp/fsharp/blob/master/src/fsharp/fsi/fsi.fs at line 1690 — John Palmer
Good idea, i had a brief look at fsi.fs but i was put off by the sheer mass of code. i will take another look at this. — user1158550
I'll delete my answer David as it may give people the mistaken impression that your question has been answered. :-) — Onorio Catenacci
For the Erlang feature, I don't think you can get around without something like shadow copying of assemblies and loading/unloading via appdomain. Perhaps MEF would be useful. Also see: bartdesmet.net/blogs/bart/archive/2006/07/29/4146.aspx — David Grenier

user1604593 user1604593 · Accepted Answer · 2013-11-21T00:02:09

I don't know if this would work in FS but in ML you can load a master file that loads all your files in your project and then executes any code that you need to use to knit them together and runs your application. To see an example of a massive app run from inside of a REPL look at the Isabelle/HOL site at the Cambridge laboratory of Computational Science http://www.cl.cam.ac.uk/research/hvg/Isabelle/installation.html. After downloading the app look in the src code directory for any file called root.ml. There will be half a dozen of them that control various levels of implementation. This is recursive because a top level file can call a file in several sub-directories that loads that particular sub-feature. This allows targeting your application to various scenarios depending on which top level file is executed.

F# 'modular' scripting

2 Answers