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Tuple Field Projection

rascal-0.40.17

Synopsis​

Projection of tuple.

Syntax​

Exp < Field₁, Field₂ ... >

Description​

Exp should evaluate to a tuple or relation, and Fieldᵢ should be a field name or an integer constant that refers to elements in the order in which they occur in the original value (counting from 0).

Examples​

Suppose we have a relation with traffic information that records the name of the day, the day number, and the length of the traffic jams at that day.

rascal>rel[str day, int daynum, int length] traffic = 
>>>>>>>{<"mon", 1, 100>, <"tue", 2, 150>, <"wed", 3, 125>, <"thur", 4, 110>, <"fri", 5, 90>};
rel[str day,int daynum,int length]: {
  <"thur",4,110>,
  <"mon",1,100>,
  <"fri",5,90>,
  <"wed",3,125>,
  <"tue",2,150>
}
rascal>traffic<length,daynum>
rel[int length,int daynum]: {
  <110,4>,
  <125,3>,
  <90,5>,
  <150,2>,
  <100,1>
}
rascal>traffic<2,day>
rel[int length,str day]: {
  <110,"thur">,
  <125,"wed">,
  <90,"fri">,
  <150,"tue">,
  <100,"mon">
}

for every tuple we can use the same notation

rascal>import IO;
ok
rascal>for (tup <- traffic) {
>>>>>>>  println(tup<2,day>);
>>>>>>>}
<110,"thur">
<100,"mon">
<90,"fri">
<125,"wed">
<150,"tue">
list[void]: []

Field projection thus selects parts from a larger value that has a fixed number of parts. The selection is based on position and not on value and can be used to completely reorder or remove the parts of a larger value.