Question

I want a function that gets the type of a value at runtime. Example use:

(get-type a)

where a has been defined to be some arbitrary Scheme value.

How do I do this? Or do I have to implement this myself, using a cond stack of boolean?, number? etc. ?

Answer 1

In Scheme implementations with a Tiny-CLOS-like object system, you can just use class-of. Here’s a sample session in Racket, using Swindle:

$ racket -I swindle
Welcome to Racket v5.2.1.
-> (class-of 42)
#<primitive-class:exact-integer>
-> (class-of #t)
#<primitive-class:boolean>
-> (class-of 'foo)
#<primitive-class:symbol>
-> (class-of "bar")
#<primitive-class:immutable-string>

And similarly with Guile using GOOPS:

scheme@(guile-user)> ,use (oop goops)
scheme@(guile-user)> (class-of 42)
$1 = #<<class> <integer> 14d6a50>
scheme@(guile-user)> (class-of #t)
$2 = #<<class> <boolean> 14c0000>
scheme@(guile-user)> (class-of 'foo)
$3 = #<<class> <symbol> 14d3a50>
scheme@(guile-user)> (class-of "bar")
$4 = #<<class> <string> 14d3b40>