occurs check

gistfile1.txt

Prelude> let x = x x

<interactive>:2:9:
    Occurs check: cannot construct the infinite type: t1 ~ t -> t1
    Relevant bindings include x :: t -> t1 (bound at <interactive>:2:5)
    In the expression: x x
    In an equation for ‘x’: x = x x
    
    
# let rec x = x x;;
Error: This expression has type 'a -> 'b
       but an expression was expected of type 'a
       The type variable 'a occurs inside 'a -> 'b
       
       
> let rec x = x x;;

  let rec x = x x;;
  ------------^^^

/Users/sean/Library/Developer/Xcode/DerivedData/lldb-ahryjcvbkbsloweweleakmzjotns/Build/Products/DebugClang/stdin(1,13): error FS0001: Type mismatch. Expecting a
    'a -> 'b
but given a
    'b
The resulting type would be infinite when unifying ''a' and ''b -> 'a'