=quote= To evaluate a combination, do the following: 1. Evaluate the subexpressions of the combination. 2. Apply the procedure that is the value of the leftmost subexpression (the operator) to the arguments that are the values of the other subexpressions (the operands). =question= can someone please explain how to apply this combination (+ 1 2 3) according to the above rule?

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(+ 1 2 3) is primitive expression, which will be evaluated as 1+2+3. But to evaluate a combination(nested expression) this rule will apply. Like (+ (*1 2) 3). 1) first evaluate the sub expression (* 1 2 ) 2) (+ 2 3) - primitive expression.solved directly ans 5

thanks @jagan but according to =quote= 1.1.1 Expression ... One kind of primitive expression you might type is a number. ... Expressions representing numbers may be combined with an expression representing a primitive procedure (such as + or *) to form a compound expression that represents the application of the procedure to those numbers. For example: (+ 137 349) ... Expressions such as these, formed by delimiting a list of expressions within parentheses in order to denote procedure application, are called combinations. =reply= so i think (+ 1 2 3) is a combination. because each of the + 1 2 3 is the primitive expression. The ( and ) is the means of combination.

Hi @ajiwo, I read again 1.1.1 Expressions section and I think that (+ 1 2 3) is a combination just because it is a list of expressions within parentheses, that is to say that is not necessary that the list of expressions were formed only by primitives expressions to be a combination. Non-primitive expressions within parentheses also form combinations.

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