ocaml - Why is this weakly polymorphic -

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After

I'm starting to learn OCaml and was trying to do some practices: 

  # left xy = x ;; Wall left: 'a - & gt; 'B - & gt; 'A = & lt; Funny & gt; #before = list.fold_right left ;; Valve first: '_a list - & gt; '_a - & gt; '_a = & lt; Funny & gt;   
 Why completely weak polymophistically  first    
 
 
 This price restriction is  first  is not a value, it is a function app.  
 To get a fully polarized version, use the Eta extension:  
  leave xy = x ;; Wall left: 'a - & gt; 'B - & gt; 'A = & lt; Funny & gt; # First A B = list.fold_right left a b ;; First Val: 'A List - & gt; 'A - & gt; 'a = & lt; Funny & gt;   
 @ivg explains, this is a commonly asked OCaml question.  
  Update   
 Here is a function that is unsafe for generalization:  
  #fx = ref x ;; val f: 'a - & gt; 'A ref = & lt; Funny & gt; # F [] ;; -: '_a list ref = {content = []}   
 If you have the result type  that play' a list referee  you can make the code go wrong ( I tried to do it).  
 Here is a partial application that is unsafe for generalization:  
  # gx = let z = ref x fun () -> Z ;; Val g: 'a - & gt; Unit - & gt; 'A ref = & lt; Funny & gt; # G [] ;; -: unit - & gt; '_a list ref = & lt; Funny & gt;   
 If you type the result  the entity that plays -> gt; 'A list ref you can wrong this code (i tried it).

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