Explain the ' notation in F#

Question

I\'m learning F#, when I type any code in Visual Studio and run them on F# Interactive it shows me things like

val foo : x:\'a -> \'a
         


        

Answer 1

'a represents a type variable, in other words a type that is not determined yet (and doesn't have to be determined yet).

Note that this is different than a', which is a regular variable whose name is two characters: a and '. Contrary to other languages like C#, the single quote is a permitted character in F# variable names, except as the first character in the name to disambiguate from type variables above.