character vector containing information how to join data frames. Format to be according to by in full_join. Each data frame has to contain the column(s) used for joining.
a <- data.frame(what=letters[1:5],a=c(1:5))
b <- data.frame(what=letters[1:5],b=c(7:11))
c <- data.frame(what=letters[1:5],c=c(20:24))
df_list <- list(a,b,c)
list_join(df_list,by="what")