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(+ (values 1) (values 2)) is fine, and the result should be 3. But it is not the case that unneeded values are automatically ignored in a values expression; in fact the behavior of (+ 1 (values 2 3)) is unspecified in both R6RS and R7RS Scheme.

From the entry for values from R6RS 11.15 [emphasis mine]:

Delivers all of its arguments to its continuation....

The continuations of all non-final expressions within a sequence of expressions, such as in lambda, begin, let, let*, letrec, letrec*, let-values, let*-values, case, and cond forms, usually take an arbitrary number of values.

Except for these and the continuations created by call-with-values, let-values, and let*-values, continuations implicitly accepting a single value, such as the continuations of and s of procedure calls or the expressions in conditionals, take exactly one value. The effect of passing an inappropriate number of values to such a continuation is undefined.

R7RS has similar language in 6.10:

The effect of passing no values or more than one value to continuations that were not created in one of these ways is unspecified.

The reason that (+ (values 1) (values 2)) is ok is that continuations of operands in procedure calls take exactly one value. (values 1) and (values 2) each provide exactly one value for their respective continuations.

call-with-values is meant for connecting producers of multiple values with procedures which consume those values. The first argument to call-with-values should be a procedure which takes no arguments and produces values to be consumed. The second argument should be a procedure which accepts the number of values produced by the first procedure:

You have 2 shader programs, hence you have to compile both shader programs and you have to install the shader program before you draw the geometry by glUseProgram:

Vuex is for managing app state in the browser for a single user. What you are looking to do is to change the state across multiple users. This would require you leverage a websocket implementation. This would allow your backend to broadcast changes to your client side app.

As @Barmar noted, definitions are not expressions, but there are two contexts where both are allowed (cf. section 5.1. of R⁷RS, emphasis mine):

• at the outermost level of a program;
• at the beginning of a body.

A Scheme program consists of one or more import declarations followed by a sequence of expressions and definitions. Import declarations specify the libraries on which a program or library depends; a subset of the identifiers exported by the libraries are made available to the program. Expressions are described in chapter 4. Definitions are either variable definitions, syntax definitions, or record-type definitions, all of which are explained in this chapter. They are valid in some, but not all, contexts where expressions are allowed, specifically at the outermost level of a ⟨program⟩ and at the beginning of a ⟨body⟩.

That is why define is allowed in the body of a procedure definition but not in a clause’s consequent of a conditional expression since it is not a body.

By default, Geiser thinks Chez Scheme's executable is scheme, however, it's not, Chez Scheme uses chez and Mit Scheme uses scheme and mit-scheme. You need to put the following in your Emacs init file: