Hello world won't compile with "The value or constructor is not defined" - f#

Question

let prefix prefixString baseString =
prefixString + " " + baseString
prefix "Hello" "World"
With the code above I'm getting the error Stuff.fs(34,1): error FS0039: The value or constructor 'prefix' is not defined.
I'm not entirely sure why this is happening, as I'm watching a video series on F# in which literally the same code is being compiled and ran successfully. Is there something wrong with my environment?

Answer 1

In the comment, you mentioned that you are running the snippet using "run selection" command. This command runs the selected piece of code in F# Interactive which initially contains no definitions. So, if you select and run just the last line, you will get:
> prefix "Hello" "World";;
stdin(1,1): error FS0039: The value or constructor 'prefix' is not defined
This is because F# Interactive does not know what the definition of prefix is - it does not look for it automatically in your file. You can fix this by selecting everything and running all code in a single interaction, or you can first run the definition and then the last line, i.e.:
> let prefix prefixString baseString =
prefixString + " " + baseString;;
val prefix : prefixString:string -> baseString:string -> string
> prefix "Hello" "World";;
val it : string = "Hello World"
Note that when you run the first command, F# Interactive will print the type of the defined functions, so you can see what has just been defined.
The fact that F# Interactive has its own separate "state of the world" is quite important, as it also means that you need to re-run functions after you change them so that subsequent commands use the new definition.