How to compare the length of a list in html/template in golang?

From documentation,

{{if pipeline}} T1 {{end}}: If the value of the pipeline is empty, no
output is generated; otherwise, T1 is executed. The empty values are
false, 0, any nil pointer or interface value, and any array, slice,
map, or string of length zero. Dot is unaffected.

So if you want to check if the .SearchData slice/array/map is empty just use,

{{if not .SearchData}} Nothing to show {{end}}

Even your code runs fine if string "0" is replaced by int 0

{{ $length := len .SearchData }} {{ if eq $length 0 }}
    Sorry. No matching results found
{{ end }}

http://play.golang.org/p/Q44qyRbKRB