Should F# functions be placed in modules, classes, or another structure? [closed]

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I'm starting to code in F# and am calling functions from functions with functions as parameters - there are plenty of learning resources online. Now I am trying to put together the pieces into something more than just a collection of functions. Unfortunate开发者_运维问答ly I'm not finding many resources dealing with structure, design, or even how the 'bits' tie together.

I've found the namespace keyword (e.g. namespace MyOnlyNamespace) but I get a compiler error on the functions that I've placed inside the namespace:

Namespaces cannot contain values. Consider using a module to hold your value declarations.

When I add module CoolFunctions I get

Unexpected start of structured construct in definition. Expected '=' or other token

So I have a multi-part question (but please answer any part that you can)

• What is a module?
• Is it like a class (something like a VB.NET module) or is it something else altogether?
• If something else, then are there classes in F#?
• Are there other structures that I should be using instead?
• How do I declare a module?

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To give some specific recommendations about choosing between namespaces, modules abd classes in F#:

• If you're writing functions using let that are expected to be used from F#, then putting them inside a module is the best choice. This gives you API similar to List.map and other basic F# functions.

  Regarding naming, you should use camelCase unless you expect C# users to call the functions too. In that case, you should use PascalCase (and note that module will be compiled to a static class).

• If you're writing type delcarations, then these should generally be placed in a namespace. They are allowed inside modules too, but then they'll be compiled as nested classes.

• If you're writing F# classes, then they should be placed in namespaces too. In generall, if you're writing F# code that will be called by C#, then using classes is the best mechanism as you get full control of what the user will see (F# class is compiled to just a class).

If you have a file, it can either start with namespace Foo.Bar or module Foo.Bar, which places all code in the file inside a namespace or a module. You can always nest more modules inside this top-level declaration. A common pattern is to start with a single namespace and then include some type and module declarations in the file:

namespace MyLibrary

type SomeType = 
  // ...

module SomeFuncs = 
  let operation (st:SomeType) = // ...

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concerning the design of F# components there is a very good draft online.

JPalmer allready pointed you to the syntay problems but I think some other questions deserve more:

What is a module?

Yes JPalmer is right - modules are compiled into static classes but do we really care inside F#? IMHO you should use more modules than classes when programming in F#. In OOP you define your classes and the methods within. In FP you define simple types (without behaviour) and a bunch of functions to transform them. And the natural place to collect those functions is the module.

Is it like a class (something like a VB.NET module) or is it something else altogether?

A VB module is indeed a good comparision.

If something else, then are there classes in F#?

Yes you can use classes in F# - it's a complete .net languague and .net is OOP. You can do practically everything in F# you could do in C# of VB.net (only certain cases generic constraints can be a pain)

Are there other structures that I should be using instead? No - collect your functions into modules but of course use records and abstract data-types for your data.

How do I declare a module?

Have a look at the online docs: Modules (F#) - there you will find everything you need.

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What is a module:

A module is compiled down to a static class. But I think of modules as being analogous to namespaces in C#

there are classes in F# - use

 type SomeType(constructor,args) = 
      ....

If you have

namespace Name
module Mod
....

this won't compile - as you know, you can use a few alternatives

module Namespace.Module

as the first line in the file

or

namespace Name
module Mod =
    ....