我写两个yield时为什么F＃会抱怨呢！ 在一排？

Question

This function concats two lists together:

let append = fun a b -> [ yield! a
                          yield! b ]


append [1;2;3] [4;5;6]

val it : int list = [1; 2; 3; 4; 5; 6]

However, if I remove the space at the beginning of the list brackets in the function (like this [yield! ... ] ), it no longer works.

Also if I do the followings it complains:

[ yield! a yield! b ]

// or this
[ yield! a
yield! b]

The second yield! must be right under the first otherwise it complains. Why? I understand how yield! works, but this seems a little weird to me that the syntax must be written exactly like this in this example.

Answer 1

In general, F# allows you to use indentation or explicit syntax in a number of places, including sequence expressions. You can put both yield! constructs on a single line by adding a semicolon:

let append a b = 
  [ yield! a; yield! b ]

If you are using indentation, then F# requires the statements of sequence expression to be aligned - so your yield! constructs have to start at the same offset. However, you do not have to indent them as far as in your first version. You can write:

let append a b = 
  [ yield! a
    yield! b ]

Another, also valid, alternative syntax (which I personally do not find that nice, but which works too) is to put the opening [ on previous line and the closing ] on a new line:

let append a b = [ 
  yield! a
  yield! b 
]

Note that I also replaced your let append = fun ab -> with an inline function definition let append ab - you can put the parameters immediately after the function name, rather than creating an explicit function using fun .