Byte size of a type in a macro (i.e. can't call `sizeof` in a macro)

Hi there, I recently starting using Crystal and I am loving it so far.

I have one issue that I can’t solve at the moment: is it possible to get the byte size of a type within a macro? When I call, e.g. sizeof(Int32) it tells me I can’t do this.

Here’s a simplified version of what I am trying to do:

class Example
  macro fields(*fs)
     {% offset = 0 %}
     {% for f in fs %}
     def {{f.var}}
        # calculate value for field based on offset
    {% offset += sizeof(Int32) %} # I want this to be f.type but that doesn't work
    {% end %}

class SubExample < Example
   fields(foo : Int32)

When I run this I get:

Error: can't execute SizeOf in a macro

Is there an alternative method I can use?

Many thanks for your help!