ListRelation

rascal-0.40.17

Synopsis

List relations are lists of tuples with relational calculus operators defined on them.

Syntax

[ < Exp₁₁, Exp₁₂, ... > , < Exp₂₁, Exp₂₂, ... > , ... ]

Types

Exp₁₁	Exp₁₂	...	{ < Exp₁₁, Exp₁₂, ... > , ... }
T₁	T₂	...	lrel[T₁, T₂, ... ]

Description

A list relation is a list of elements with the following property:

• All elements have the same static tuple type.

ListRelations are thus nothing more than lists of tuples, but since they are used so often we provide a shorthand notation for them. ListRelations are represented by the type lrel[T₁ L₁, T₂ L₂, ... ], where T₁, T₂, ... are arbitrary types and L₁, L₂, ... are optional labels. It is a shorthand for list[tuple[T₁ L₁, T₂ L₂, ... ]].

An n-ary list relation with m tuples is denoted by [< E₁₁, E₁₂, ..., E₁ₙ>,< E₂₁, E₂₂, ..., E₂ₙ>, ..., < Eₘ₁, Eₘ₂, ..., Eₘₙ>], where the Eᵢⱼ are expressions that yield the desired element type Tᵢ.

Since list relations are a form of list all operations (see List) and functions (see List) are also applicable to relations.

The following additional operators are provided for list relations:

• CartesianProduct
• Composition
• FieldSelection
• Join
• ReflexiveTransitiveClosure
• Subscription
• TransitiveClosure

There are also library functions available for list relations.

Examples

rascal>[<1,10>, <2,20>, <3,30>]
lrel[int,int]: [
  <1,10>,
  <2,20>,
  <3,30>
]

Instead of lrel[int,int] we can also give list[tuple[int,int]] as type of the above expression remember that these types are interchangeable.

rascal>[<"a",10>, <"b",20>, <"c",30>]
lrel[str,int]: [
  <"a",10>,
  <"b",20>,
  <"c",30>
]
rascal>[<"a", 1, "b">, <"c", 2, "d">]
lrel[str,int,str]: [
  <"a",1,"b">,
  <"c",2,"d">
]