Lets parse the definition of eval.

text		syntactic category
`let rec eval : type a. a term -> a = ... (eval x)`	is a	definition
`eval : type a. a term -> a = ... (eval x)`	is a	let-binding
`eval : type a. a term -> a`	is not a	pattern

for there are no parens! We have a parsing dead end. Backtrack! I can find no rule for typexpr that begins “type”.

The shortest admissible top level construct with this construct is

let e :type a. int = 3; e;;

lack of parens