Surround return type annotations with white space [F# style guide]

[cmeeren]

Issue created from fantomas-online

Code

let expensiveToCompute : int = 0
let myFun (a: decimal) b c : decimal = a + b + c

Result

let expensiveToCompute: int = 0
let myFun (a: decimal) b c: decimal = a + b + c

Problem description

F# style guide:

In a let-bound function or value type annotation (return type in the case of a function), use white space before and after the : symbol:

// OK
let expensiveToCompute : int = 0 // Type annotation for let-bound value
let myFun (a: decimal) b c : decimal = a + b + c // Type annotation for the return type of a function
// Bad
let expensiveToComputeBad1:int = 1
let expensiveToComputeBad2 :int = 2
let myFunBad (a: decimal) b c:decimal = a + b + c 

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas Master at 01/29/2021 10:31:35 - c1a4fe6

Default Fantomas configuration

[nojaf]

Please try and set fsharp_space_before_colon to true.

[cmeeren]

No dice, that produces this:

let expensiveToCompute : int = 0
let myFun (a : decimal) b c : decimal = a + b + c

While the return type and let annotation are correct, a : decimal is not. This is also a violation of the F# style guide, which says a: decimal.

[nojaf]

I see, thanks for clarifying this.

[knocte]

@nojaf This will basically change all the type declarations of all code, will fsharp_space_before_colon=false override this new behaviour?

[nojaf]

No it won't. Both style guides dictate that the return type colon needs a space before it so it is outside the control of a setting.

[knocte]

Mmm I see, then the setting should probably be renamed to fsharp_always_space_before_colon 🤔