失敗
import System.Environment main::IO() main= do args<-getArgs argsl<-length args --Couldn't match expected type ‘IO a0’ with actual type ‘Int’ (print.show) argsl
成功
import System.Environment main::IO() main= do args<-getArgs let argsl=length args (print.show) argsl
import System.Environment main::IO() main= do argsl<-length<$>getArgs (print.show) argsl