What does #light mean?


If you see any F# code listing, you will see that almost all of them start with the command #light. Right now, this is a requirement. The first line in any F# file must have #light. After reading about the language, I grokked that it allowed for a shorter syntax than the alternative, #light “off”. Here’s what I’ve been able to figure out so far. From Development in a Blink, I see this:

The #light declaration makes whitespace significant. Allowing the developer to omit certain keywords such as in, ;, begin, and end.

For those of you that have used Python, this makes sense. Indentation gets to have meaning when #light is on. Blocks of code are physically aligned, so indicating where something starts and stops is redundant and, potentially, useless. So, what happens when we turn #light off? Let’s take the Name type developed in a recent post. With #light, the class looks like this:

 

#light

 

namespace SomeNamespace

type Name(fname : string) =

    let mutable fName  = fname

    member this.FName with get() = fName

                                    and set(x) = fName <- x

So, what did #light buy us? Well, it meant that we couldn’t write the following, equivalent code:

 

#light “off”

 

namespace SomeNamespace

type Name(fname : string) =

class

let mutable fName  = fname

member this.FName with get() = fName

and set(x) = fName <- x

end

The difference, in case you missed it, is that class and end HAD to be declared. Furthermore, the indentation/whitespace was no longer significant. This let’s me write code that is harder to visually parse. And yes, I could have indented the code to make it more readable:

 

#light “off”

 

namespace SomeNamespace

    type Name(fname : string) =

    class

        let mutable fName  = fname

        member this.FName with get() = fName

            and set(x) = fName <- x

    end

 

I think I’ll be using #light since most examples on the web stick with this syntax. Being the odd man out won’t help me or anyone else who eventually needs to use my code. Good habits start now. #light also let’s me write less code. I have a fairly consistent ratio of lines of code/defects. If I can avoid some little details and lines of code, I will have fewer mistakes.

  1. Leave a comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: