Question

Something simple as this:

Welcome to DrScheme, version 4.2.3 [3m].
Language: Lazy Scheme; memory limit: 128 megabytes.

> (let ((x 2) (y 10))
   (+ x y))

#<promise>

> 

I press enter for the let expression, and it gives me the #<promise>. What am I doing wrong?

Answer 1

It says Language: Lazy Scheme;. I’m sure this means that you’re using a variant of scheme that runs lazily – i.e. it doesn’t evaluate an expression until the result is required. The way scheme will manage this internally will be by using scheme’s promise mechanism – instead of returning the result of an expression, a promise to calculate the result later is returned. You should be able to get the result explicitly by calling force against this promise.

Here are a couple of references:

• Wikipedia article on lazy evaluation.
• Scheme r5rs on force and delay.

A non-lazy scheme will behave in the way you expect.

HTH