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Reducer

rascal-0.28.2

Synopsis

Reduce generated values to a single value.

Syntax

( InitExp | RedExp | Gen₁, Gen₂, ... )

Description

Reducers are comprehesions (see list comprehension, set comprehension) that construct any type of value by an iterating rather than just a list or a set.

Every reducer (as above) is equivalent to the following code:

it = _InitExp_;      ❶  
for(_Gen₁_, _Gen₂_, ... ) ❷
it = _RedExp_; ❸
it; ❹

and is executed as follows:

  1. A fresh variable, always named it, is initialized with InitExp. We call the variable it since we use it to initialize the reducer, to make changes to it, and to return it as result.
  2. A for loop iterates over all values produced by the generators Gen₁, Gen₂, ... .
  3. In the body of the loop, variable it is updated to reflect a new reduced value. Note that it itself and variables introduced in Gen₁, Gen₂, ... may occur in RedExp.
  4. The final value of it is the result of the reducer.

Examples

rascal>L = [1, 3, 5, 7];
list[int]: [1,3,5,7]
rascal>(0 | it + e | int e <- L);
int: 16
rascal>(1 | it * e | int e <- L);
int: 105

Benefits

  • A reducer resembles the fold function found in most functional languages.