#/._?NumberQ->0/.x_String:>StringReplace[x,_->" "]&
StringReplace[#,_->" "]~Check~#&//@#/._?NumberQ->0&

#/._?NumberQ->0

.../.x_String:>StringReplace[x,_->" "]

StringReplace[#,_->" "]~Check~#&//@...

InstallOtherMethod(Zero,[IsChar],c->' ');
InstallOtherMethod(Zero,[IsList],l->List(l,Zero));

gap> Zero([[["H"],"e",1,1,0],[[-3],"arth"]]);
[ [ [ " " ], " ", 0, 0, 0 ], [ [ 0 ], "    " ] ]

f:=function(x)if IsInt(x)then return 0;elif IsList(x)then return List(x,f);fi;return' ';end;

f=->a{a.map{|x|x*0==0?0:x*0==""?x.gsub(/./," "):f[x]}}

class N a where n::a->a
instance N Int where n _=0
instance N Char where n _=' '
instance N a=>N[a]where n=map n

*Main> n (42::Int)
0
*Main> n "hi"
"  "
*Main> n ([[1,2],[3,4,5]]::[[Int]])
[[0,0],[0,0,0]]
*Main> n ["hello","world!"]
["     ","      "]