Lisp why use lambda

The power of this approach is that we can pass functions as arguments to other functions, and write functions that return functions as their result. The funcall function we just used is a function that takes another function as an argument, takes one or more arguments as appropriate for the function , and invokes that function, passing it the arguments. Another way to use this is using the apply function.

It takes a function and a list of arguments for it, and returns the result of applying that function to each of the arguments. For example:. The funcall function is similar to apply , but the restriction on the final argument is lifted:.

Both apply and funcall are applicative operators. Probably the most frequently used applicative operator is mapcar , which applies a function to each element of a list, one at a time, and returns a list of the results. These applicative operators do not require named functions -- we can also use lambda expressions. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog.

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Now live: A fully responsive profile. Linked 1. Related Hot Network Questions. Question feed. Stack Overflow works best with JavaScript enabled. Accept all cookies Customize settings. I could have added a Nil sentinel to the Expr data type and used it instead of Lf "" , but the code seemed more honest without it. The f function is also a good place to handle label , and some Lisp shorthand:. Locally, we give the name g to the evaluation function for a given environment:.

For labels, we add a binding to the environment, then evaluate the newly bound function on the given arguments. For lambda , we modify the environment accordingly before recursing. Otherwise we expect the left child of the root to be a leaf. If it has a binding in the environment, we recursively evaluate it on the rest of the tree. If not, then we do evaluate the arguments first before calling f. Again we see the overheads incurred by using a non-list data structure in Haskell.

In my initial list-based code, I could simply use the default map instead of mapL , and fromTree would be unneeded. We use a list to store bindings. We permanently augment the global environment when the eval function returns a Label. We preload definitions of the form defun cadr x cdr car x from caar to cddddr. For this webpage, we set up one button to run the program supplied in the input textarea and place the results in the output textarea.

Another button fills the input textarea with a predefined program. The command-line variant of our program provides more immediate gratification, printing results on every newline if it terminates a valid expression. Graham calls it The Surprise. By adding a handful of primitives to lambda calculus, we can write a self-interpreter that fits on a page.

This is impressive, and more elegant than universal Turing machines. Haskell is a fashionable five-star high-tech luxurious language, but stripping away its contemporary furnishings reveals a humble core surprisingly similar to Lisp. In this small example we see various sweeteners: the off-side rule ; pattern matching; guards; infix and prefix notation; concise notation for lists.

It is best to avoid ' in most instances because it is "mostly" unnecessary and makes your code more verbose. There are a few exceptions when some form of quoting is necessary see Example 4 below. The basic concepts are the same in both dialects. Functions are first class objects e. Anonymous functions e. Read on. I promise to make this succinct. Now, you need a rough understanding of how Lisp evaluates S-expressions parenthetical expressions. Every S-expression is evaluated roughly as follows:.

Keep in mind that the purpose of quoting is to prevent evaluation. When Lisp evaluates the CAR of. The question is thus when to quote functions or symbols holding functional definitions. See the following examples to gain a better understanding:. You can assign a value to its value cell if you like using. The function apply is quite useful in recursion. Suppose you want to sum all the elements in a list.

Remember the evaluation rules I outlined above. Lisp evaluates the symbol apply by retrieving the value stored in its function cell.

Actually, if we had not set its value cell to "I am the plus function," Lisp would have retrieved nil, which is NOT a function, as required by apply. Yes, there is. You can just evaluate the following code:. This is achieve as follows:. You get around this by using funcall, which tells Lisp to use the value in the symbol's value cell as a functional definition. You do this using "funcall. You could avoid having using funcall by placing the lambda form in the symbol's function cell using fset :.

This code block will return 20 because lisp finds a functional definition in AFunc 's function cell. A typical solution is to define a function valid only inside the scope of the main one.