Question

This is an example in SICP, I enter it but it has an error.

(define (sum term a next b)
  (if (> a b)
      0
      (+ (term a)
         (sum term (next a) next b))))

This is the error:

function call: expected a function after the open parenthesis, but found a variable

Answer 1

If you are using DrRacket, you may be running DrRacket under the “Beginner Student Language” mode. At the introductory CS level, that particular language level mode disables higher-order functions. SICP, on the other hand, assumes a language level that allows higher-order procedures.

Assuming this is the case, you can fix this by adjusting DrRacket so it uses a language level that allows higher-order procedures such as #lang racket.

See Choosing a Language: switch DrRacket over to “Use the language declared in the source”, and make the top line of your program:

#lang racket

That should then let you write your function.

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By the way there is specialized support for SICP for DrRacket; if you’re going to go through SICP, you probably want to use this. You can enable this support by changing the top #lang line to:

#lang planet neil/sicp

It’ll take a little time the first time you run a program with this, as it automatically installs the support software; subsequent use of the package should be fast.