lib/generators: floats are not supported in mkValueStringDefault

They are cut off after a few decimal places; we cannot in good faith
define a default string representation with that.

lib/generators.nix

@@ -46,7 +46,10 @@ rec {
     else if isList     v then err "lists" v
     # same as for lists, might want to replace
     else if isAttrs    v then err "attrsets" v
+    # functions can’t be printed of course
     else if isFunction v then err "functions" v
+    # let’s not talk about floats. There is no sensible `toString` for them.
+    else if isFloat    v then err "floats" v
     else err "this value is" (toString v);