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Natural Language Processing with Python Phần 9

Chúng tôi có một cái tên đặc biệt cho kết quả của sự ràng buộc các biến trong một biểu thức: λ-trừu tượng. Khi bạn lần đầu tiên gặp λ-tóm tắt, nó có thể được khó để có được một ý thức trực giác về ý nghĩa của chúng. | Ă is a binding operator just as the first-order logic quantifiers are. If we have an open formula such as 33a then we can bind the variable x with the Ă operator as shown in 33b . The corresponding NLTK representation is given in 33c . 33 a. walk x chew_gum x b. Ăx. walk x chew_gum x c. x. walk x chew_gum x Remember that is a special character in Python strings. We must either escape it with another or else use raw strings Section 3.4 as shown here lp nltk.LogicParser e lp.parse r x. walk x chew_gum x e LambdaExpression x. walk x chew_gum x e.free set print lp.parse r x. walk x chew_gum y x. walk x chew_gum y We have a special name for the result of binding the variables in an expression Ă-abstraction. When you first encounter Ă-abstracts it can be hard to get an intuitive sense of their meaning. A couple of English glosses for 33b are be an x such that x walks and x chews gum or have the property of walking and chewing gum. It has often been suggested that Ă-abstracts are good representations for verb phrases or subjectless clauses particularly when these occur as arguments in their own right. This is illustrated in 34a and its translation 34b . 34 a. To walk and chew gum is hard b. hard x. walk x chew_gum x So the general picture is this given an open formula Ọ with free variable x abstracting over xyields a property expression Ăx.ọ the property of being an x such that Ọ. Here s a more official version of how abstracts are built 35 If a is of type T and x is a variable of type e then x.a is of type e ĩ . 34b illustrated a case where we say something about a property namely that it is hard. But what we usually do with properties is attribute them to individuals. And in fact if Ọ is an open formula then the abstract Ăx.ọ can be used as a unary predicate. In 36 33b is predicated of the term gerald. 36 x. walk x chew_gum x gerald Now 36 says that Gerald has the property of walking and chewing gum which has the same meaning as 37 . 37 walk gerald chew_gum gerald 10.4