I took some typical code from vm.nim, to show what kind of repetitive coding is still widely
common practice (not just in the case of Nim):

    of opcAddFloat:
      decodeBC(rkFloat)
      regs[ra].floatVal = regs[rb].floatVal + regs[rc].floatVal
    of opcSubFloat:
      decodeBC(rkFloat)
      regs[ra].floatVal = regs[rb].floatVal - regs[rc].floatVal
    of opcMulFloat:
      decodeBC(rkFloat)
      regs[ra].floatVal = regs[rb].floatVal * regs[rc].floatVal
    of opcDivFloat:
      decodeBC(rkFloat)
      regs[ra].floatVal = regs[rb].floatVal / regs[rc].floatVal

Here is pseudo code that would prevent this:
    oftemplate opc<when>Float(op: untyped):
      when Add: `+`
      when Sub: `-`
      when Mul: `*`
      when Div: `/`
      decodeBC(rkFloat)
      regs[ra].floatVal = op(regs[rb].floatVal, regs[rc].floatVal)

1) Using "oftemplate" to differentiate from normal of, but could in the end merge with normal "of" I think, if something like that became popular enough
2) I'm not sure on the <when> tag-like-syntax, maybe something better can be found
3) This is just an idea, please do not get offended by an idea!
4) This can already mostly be implemented with a regular Nim template. The advantage here is that the template gets cleanly embedded into the regular case-of structure. You would need this template only once anyways, so way create it outside of the case-of structure?