julia> dostuff(a::Real) = println(2*a);

julia> dostuff(2.0);
4.0

julia> dostuff(2);
4

julia> dostuff("2");
ERROR: MethodError: no method matching dostuff(::String)
Closest candidates are:
  dostuff(::Real) at REPL[1]:1
Stacktrace:
 [1] top-level scope at none:0

julia> domorestuff(a::Float64) = printlnt(2*a);

julia> domorestuff(2);
ERROR: MethodError: no method matching domorestuff(::Int64)
Closest candidates are:
  domorestuff(::Float64) at REPL[5]:1
Stacktrace:
 [1] top-level scope at none:0