haskell couldnt match expeted type : On this code there's no way to do it polymorphic:

    data NumericExpr e = Var e | Const e | Times [e] | Div e e deriving (Show,Read)
    
    readCommand:: a -> b
    readCommand entrada = Var 3


it gives me a big error which I can't copy:
couldn't match expected type b with actual type NumericExpr e0 b is a rigid type variable bound by the type signature for readcommand :: a -> b 
