my_summarise() is a custom function that automatically choose between sym() and enquo() for input type "character" and ^quosure

## This works!
my_summarise <- function(df, var){
  if(inherits(substitute(var), "character")){
    var = sym(var)
  }
  else if(inherits(substitute(var), "name")){
    var = enquo(var)
  }
  df %>% summarise(avg = mean(!!var))
}

identical(my_summarise(iris, Sepal.Length),
          my_summarise(iris, 'Sepal.Length'))

# TRUE

######################

## Thought substitute() was equivalent to enquo() but it's not. Replacing substitute() with enquo()

my_summarise2 <- function(df, var){
  if(inherits(enquo(var), "character")){
    var = sym(var)
  }
  else if(inherits(enquo(var), "name")){
    var = enquo(var)
  }
  df %>% summarise(avg = mean(!!var))
}

identical(my_summarise2(iris, Sepal.Length),
          my_summarise2(iris, 'Sepal.Length'))

# object 'Sepal.Length' not found

#####################

## Replacing enquo() with enexpr() WORKS !
my_summarise3 <- function(df, var){
  if(inherits(enexpr(var), "character")){
    var = sym(var)
  }
  else if(inherits(enexpr(var), "name")){
    var = enquo(var)
  }
  df %>% summarise(avg = mean(!!var))
}

identical(my_summarise3(iris, Sepal.Length),
          my_summarise3(iris, 'Sepal.Length'))

# TRUE