Name Rascal/Expressions/Comprehensions/Enumerator

Synopsis Enumerate all values in a given subject value.

Syntax Pattern <- Exp

Description An enumerator generates all the values in a given subject value. For elementary types (bool, int, real, num, loc, datetime, str) this is just a singleton. For composite types (list, set, map, tuple, rel, node) the values of their elements, respectively, their direct children are enumerated. An enumerator is evaluated as follows:

• Expression Exp is evaluated and may have an arbitrary value V.
• The elements of V are enumerated one by one.
• Each element value is matched against the pattern Pattern. There are two cases:
  • The match succeeds, any variables in Pattern are bound, and the next generator in the comprehension is evaluated. The variables that are introduced by an enumerator are only available to generators that appear later (i.e., to the right) in the comprehension. When this enumerator is the last generator in the comprehension, the contributing expressions of the comprehension are evaluated.
  • The match fails, no variables are bound. If V has more elements, a next element is tried. Otherwise, a previous generator (i.e., to the left) is tried. If this enumerator is the first generator in the comprehension, the evaluation of the comprehension is complete.

Type information is used to check the validity of an enumerator and guard you against mistakes. An impossible enumerator like

int N <- {"apples", "oranges"}

will be flagged as an error since the pattern can never match.

Examples Here are some examples of enumerators:

• int N <- {1, 2, 3, 4, 5}.
• str K <- KEYWORDS, where KEYWORDS should evaluate to a value of set[str].
• <str K, int N> <- {<"a",10>, <"b",20>, <"c",30>}.
• <str K, int N> <- FREQUENCIES, where FREQUENCIES should evaluate to a value of type rel[str,int].
• <str K, 10> <- FREQUENCIES, will only generate pairs with 10 as second element.
• int N <- 17, will only generate the value 17.

Here are examples of enumerators in action:

rascal>[ N * N | int N <- [1, 2, 3, 4, 5] ];
list[int]: [1,4,9,16,25]
rascal>{<N, K> | <str K, int N> <- {<"a",10>, <"b",20>, <"c",30>}};
rel[int,str]: {
  <10,"a">,
  <20,"b">,
  <30,"c">
}

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