Add whitespace formatting guidelines

docs/fsharp/style-guide/formatting.md

@@ -1,7 +1,7 @@
 ---
 title: F# code formatting guidelines
 description: Learn guidelines for formatting F# code.
-ms.date: 05/14/2018
+ms.date: 11/26/2018
 ---
 # F# code formatting guidelines
 
@@ -25,6 +25,63 @@ When indentation is required, you must use spaces, not tabs. At least one space
 
 That said, indentation of programs is a subjective matter. Variations are OK, but the first rule you should follow is *consistency of indentation*. Choose a generally accepted style of indentation and use it systematically throughout your codebase.
 
+## Formatting white space
+
+F# is white space sensitive. Although most semantics from white space are covered by proper indentation, there are some other things to consider.
+
+### Formatting operators in arithmetic expressions
+
+Always use white space around binary arithmetic expressions:
+
+```fsharp
+let subtractThenAdd x = x - 1 + 3
+```
+
+Unary `-` operators should always have the value they are negating immediately follow:
+
+```fsharp
+// OK
+let negate x = -x
+
+// Bad
+let negateBad x = - x
+```
+
+Adding a white-space character after the `-` operator can lead to confusion for others.
+
+In summary, it's important to always:
+
+* Surround binary operators with white space
+* Never have trailing white space after a unary operator
+
+The binary arithmetic operator guideline is especially important. Failing to surround a binary `-` operator, when combined with certain formatting choices, could lead to interpreting it as a unary `-`.
+
+### Surround a custom operator definition with white space
+
+Always use white space to surround an operator definition:
+
+```fsharp
+// OK
+let ( !> ) x f = f x
+
+// Bad
+let (!>) x f = f x
+```
+
+For any custom operator that starts with `*`, you'll need to add a white space to the beginning of the definition to avoid a compiler ambiguity. Because of this, it's recommended that you simply surround the definitions of all operators with a single white-space character.
+
+### Surround function parameter arrows with white space
+
+When defining the signature of a function, use white space around the `->` symbol:
+
+```fsharp
+// OK
+type MyFun = int -> int -> string
+
+// Bad
+type MyFunBad = int->int->string
+```
+
 ## Formatting blank lines
 
 * Separate top-level function and class definitions with two blank lines.