wiki:RefactoringSteps/ListComprehensions

Transform list comprehension

Turn lists:map, lists:foreach and lists:filter calls into list comprehension syntax, or do it backwards.

The two main cases of the transformation:

  • lists:map/2 or lists:filter/2 or lists:foreach/2 to list comprehension
    • The transformation is applied only if lists:map/2 or lists:filter/2 or lists:foreach/2 is selected
    • If the first parameter is an explicit or an implicit fun expression the arity of the function must be equal to 2
    • If the first parameter is not a fun it is not checked what it really is
    • It is not checked whether the second parameter really a list is
  • list comprehension to lists:map/2 and/or lists:filter/2
    • The transformation does not supports list comprehensions that contain more than one list generator
    • The result is a lists:filter/2 or a lists:map/2 or a composition of a lists:filter/2 and a lists:map/2
    • The transformation does not optimize according to unused variables
    • It is not checked whether the generator really a list is

Transform between two equivalent forms of list filtering im:

f(Xs) ->
   [X || X <- Xs, X < 5].
f(Xs) ->
   lists:filter(
      fun(X) ->
         X < 5
      end, Xs).

Side conditions

  • lists:map/2 or lists:filter/2 or lists:foreach/2 to list comprehension
    • The selection should contain an application of one of the relevant functions
    • If the first parameter is an implicit function or an explicit function, the arity has to be 1.
  • list comprehension to lists:map/2 and/or lists:filter/2
    • The list comprehension can have only one generator; transforming more complex comprehensions would likely complicate the code even more.

Transformation steps and compensations

  • lists:map/2 or lists:foreach/2 to list comprehension
    • If the first parameter is an implicit function, a new variable is created in the list comprehension and the implicit function is called with this variable.
    • If the first parameter is an explicit function
      • If it has only one clause without pattern matching and guards
        • If it has only one body the body is copied into the head of the list comprehension.
        • If it has more then one body, a begin..end expression is created.
      • If it has more then one clause or one clause with pattern matching or with guards, a case expression is created in the head of the list comprehension. The application of the case is a new variable, and the branches of the case are the clauses of the function.
    • If the first parameter is anything else, an application is created for it in the head of the list comprehension.
  • lists:filter/2 to list comprehension
    • If the first parameter is an implicit function, a new variable is created in the list comprehension and the implicit function is called with this variable.
    • If the first parameter is an explicit function
      • If the function has one clause and the function has no pattern matching and no guards and the body returns a boolean value
        • If the function has only one body then it is inserted into the list comprehension as filter(s).
        • If the function has more then one body, then the bodies are inserted into the list comprehension in a begin..end structure.
      • If the function has two clauses and the second body has no pattern matching and no guards and returns constantly false
        • If the first clause has pattern matching, it is used in the generator.
        • If the first clause has guards, they are are inserted into the list comprehension as filters.
        • If the first clause has only one body which returns boolean value, it is inserted into the list comprehension as filter(s).
        • If the first clause has more then one body, then the bodies are inserted into the list comprehension in a begin..end structure.
      • If the function has more then two clauses it is transformed into a case structure.
    • If the first parameter is everything else it is inserted into the list comprehension as an application.

  • list comprehension to lists:map/2 and/or lists:filter/2
    • Building the lists:filter/2
      • The filters are inserted a function expressions body.
      • If the generator has pattern matching then the created function expression will have two clauses where the first clause has the pattern matching the second clause is constantly false.
      • If no filters and no pattern matching occurs then no lists:filter/2 is created.
    • Building the lists:map/2
      • If the head and the pattern of the list comprehension are equals, no lists:map/2 is created.
      • Else a fun expression is created with the list comprehensions pattern and the list comprehensions head as body.
Last modified 12 years ago Last modified on Feb 19, 2012, 10:00:20 PM