What do 'T and 'U mean in F#?

Question

Such as in the http://msdn.microsoft.com/en-us/library/ee370378.aspx example List.map : ('T -> 'U) -> 'T list -> 'U list .

Sorry if this sounds simplistic, but Googling it turns up no explanations.

Thanks!

Answer 1

In this case 'T and 'U are arbitrary types.

In other cases, restrictions might be placed on 't using constraints.

Occasionally the names may be descriptive like 'key or 'value. In F#, the key is that a ' before a type makes it generic.

Similarly, types may be prefixed with a # which allows for any type which can be upcast to the given type.

More complex type constraints using inline methods can be used with a ^ before the type name allowing for member constraints which are not possible using the .Net type system and will only work with F# inline functions.

Answer 2

From the specifications:

A type of the form 'ident is a variable type. For example, the following are all variable types:

'a
'T
'Key

And for your question on why 'T and not 'a:

Note: this specification generally uses uppercase identifiers such as 'T or 'Key for user-declared generic type parameters, and uses lowercase identifiers such as 'a or 'b for compiler-inferred generic parameters.

http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/manual/spec.html

In this case, all it means is List.map : ('T -> 'U) -> 'T list -> 'U list takes in a function that converts 'T type value to 'U type, a list of 'T type values and returns a list of ''U' type values - which is, of course, what a map does.