[Error] Unit1.pas(30): Incompatible types: 'String' and 'Char',该怎么处理

[Error] Unit1.pas(30): Incompatible types: 'String' and 'Char'
var ch:string;
begin
  ch:= 'a';
  if ch in [ '0'..'9'] then ShowMessage( '是数字')
  else ShowMessage( '不是数字');
end;

[解决办法]
var
ch: Char;
[解决办法]
if ch[1] in [ '0'..'9'] then