F#冗长的语法和类定义问题

Question

I'm baffled by the fsharp-error given for following code in test.fsx :

let Kasper = "Kasper" in printfn "%A" Kasper
type student (name: string) =
  class
    member this.name = name
  end
let aStudent = student Kasper in printfn "%A" aStudent.name

which gives the following error,

% fsharpc test.fsx && mono test.exe
Microsoft (R) F# Compiler version 10.2.3 for F# 4.5
Copyright (c) Microsoft Corporation. All Rights Reserved.

/Users/sporring/Desktop/test.fsx(2,1): error FS0010: Unexpected keyword 'type' in implementation file
%

I have found the following solutions:

• move the printfn "%A" Kasper.name; to a new line
• move the whole construction let Kasper = "Kasper" in printfn "%A" Kasper below the type definition
• convert it all to lightweight syntax

So it seems, that I don't understand the scope of the let-in expression. Would someone please help me understand why the original code fails to compile? Thanks.

Answer 1

The verbose syntax of F# is odd and there is a good reason why it's discouraged :-).

The let .. in .. construct is an expression, but the top-level in a module (which is what you need when you also want to use type ) behaves differently. At the module level, you can use let to define top-level values, but this is not let .. in .. expression but rather a declaration let <pat> = <expr> . You can also use the do keyword at the top-level if you want to run some code when the module is initialized, which is written as do <expr> . So, to make your example work, you can use:

#light "off"
let Kasper = "Kasper" do printfn "%A" Kasper
type student (name: string) =
  class
    member this.name = name
  end
do let aStudent = student Kasper in printfn "%A" aStudent.name

To show more options, I write the first let as a top-level declaration, but the aStudent value is defined as a local binding in a do block.