How to create an empty environment represented by a function

Question

I came across this problem online which I found interesting, it had a decent explanation but I was confused about the solution. So given

type 'a fenv = name -> 'a 

create a value of type

'a fenv

which would be our empty env. I thought this to be something along the following

exception NotFound of name
val empty = fn name => raise NotFound name

Where it might return a Notfound name exception but I may be doing this wrong because I continue to get

Type clash: expression of type
   'a alist -> 'a alist alist
cannot have type
   'a alist alist
Toplevel input:
val (_: 'a fenv) = empty
Unbound type constructor: fenv

Sorry if this is simple still new at sml could someone explain how I would go about getting the solution?

Thanks

Answer 1

How to create an empty environment represented by a function

You could do:

type name = string
exception NotFound of name
fun empty name = raise NotFound name

I may be doing this wrong because I continue to get

Type clash: expression of type
   'a alist -> 'a alist alist
cannot have type
   'a alist alist

You do realize that your code mentions nothing of 'a alist, right? You're probably either using a REPL that has an outdated definition for type 'a fenv, or your file contains multiple definitions, one involving this 'a alist and another containing this 'a fenv.