Question

This compiler like:

let test Xf Yf = Xf + Yf

This compiler no like:

let test Xfd Yfd = Xfd + Yfd

Warning:
Uppercase variable identifiers should not generally be used in patterns, and may indicate a misspelt pattern name.

Maybe I’m not googling properly, but I haven’t managed to track down anything which explains why this is the case for function parameters…

Answer 1

I agree that this error message looks a bit mysterious, but there is a good motivation for it. According to the F# naming guidelines, cases of discriminated unions should be named using PascalCase and the compiler is trying to make sure that you don’t accidentally misspell name of a case in pattern matching.

For example, if you have the following union:

type Side = 
  | Left 
  | Right

You could write the following function that prints “ok” when the argument is Left and “wrong!” otherwise:

let foo a = 
  match a with 
  | Lef -> printfn "ok"
  | _ -> printfn "wrong!"

There is a typo in the code – I wrote just Lef – but the code is still valid, because Lef can be interpreted as a new variable and so the matching assigns whatever side to Lef and always runs the first case. The warning about uppercase identifiers helps to avoid this.