Hammar's comment is correct. See below:
*Main Text.StringTemplate> render $ setAttribute "optSet" False $ (newSTMP "OptSet: $if(optSet)$Option Is Set$else$Option Isn't Set$endif$" :: StringTemplate String)
"OptSet: Option Isn't Set"
*Main Text.StringTemplate> render $ setAttribute "optSet" True $ (newSTMP "OptSet: $if(optSet)$Option Is Set$else$Option Isn't Set$endif$" :: StringTemplate String)
"OptSet: Option Is Set"