# Rel Primer: Advanced Syntax

This Primer introduces more advanced Rel syntax.

## Introduction

This document assumes that you are familiar with basic Rel syntax, and introduces more advanced features of the Rel language.

## Grounded and Ungrounded Variables

You can begin by considering what relations Rel can be expected to compute.

A variable is grounded when it can be instantiated to a specific, finite set of values.
For example, in the expression
`x: range(1, 100, 1, x)`

, using the built-in `range`

relation,
the variable `x`

is instantiated to the 100 different values `1, 2, 3, ..., 100`

to produce the relation.
Similarly, when you write `x : minimum(1, 2, x)`

, the variable `x`

is bound to `1`

.

By contrast, if you write `x: minimum(1, x, 1)`

, there are infinitely many values of `x`

for which `minimum(1, x, 1)`

is true;
and if you write `x, y: range(1, y, 1, x)`

, there are infinitely many possible values for both `x`

and `y`

.
These variables are said to be *ungrounded*.

When the RAI query engine encounters one of these cases, you will see an error stating that the definition “contains ungrounded variable(s); hence, the rule cannot be evaluated.”

In general, if you write `range(x, y, z, result)`

, you should expect to know the values of `x, y, z`

, and say that
those values *grounded* the value of `result`

.

`@inline`

Definitions

You can still declare and use definitions that are not expected to be fully computed, by using the @inline annotation. For example:

```
// read query
@inline
def mymax[x, y] = maximum[abs[x], abs[y]]
def output = mymax[-10, -20]
```

Here, `mymax`

is not intended to be computed directly — you would not expect to enumerate all the tuples that satisfy it.
If it were not labeled as `@inline`

,
the Rel compiler would attempt to compute `mymax`

and report that `x`

and `y`

are ungrounded.

You can add the `@inline`

annotation to any relation.
If you have a large derived relation but are only interested in querying it for particular values,
it can be best to inline it, so that the system does not compute all values in that relation.
This is also the case if the rest of your code only needs a few values.

## Relations as Arguments

Relations in Rel are first-order, meaning they do not take relations as values.
But you can use @inline to define *syntactically* higher-order relations, which
can take a relation as one of their arguments.

For example:

```
// read query
@inline
def maxmin[Relation] = (max[Relation], min[Relation])
def output = maxmin[ {1; 2; 3; 4; 5} ]
```

Here you used max and min from the Standard Library, which take relations as their argument as well.

In an `@inline`

definition, variable names that correspond to relations must start with an uppercase letter,
as is the case with `Relation`

above.

This does not contradict the first-order nature of the language: When the @inline definition is expanded, the result is first-order again. Readers with experience with programming languages can think of inline definitions as macros that are expanded at compile-time. They are not directly evaluated themselves, keeping things first-order.

`argmax`

is another Standard Library utility that takes a relation `R`

as an argument,
returning the rows in `R`

that have the largest value.
For example:

```
// read query
def output = argmax[{("a", 2, 3); ("b", 1, 6) ; ("c", 10, 6)}]
```

You can use argmax on the sample data from Aggregations
to find the highest-paid players for a particular team. Here are the `plays_for`

and `salary`

relations:

`plays_for`

`salary`

```
// read query
def output = argmax[name, s : plays_for(name, "BFC") and salary(name, s)]
```

It is easy to generalize this query to get the highest-paid player for each team that participates in the `plays_for`

relation:

```
// read query
def output = team: argmax[name, s : plays_for(name, team) and salary(name, s)]
```

### Example

Here’s an example of how @inline functions can give the code a higher-order flavor.
The `x...`

syntax is explained in the Varargs section below.

```
// read query
@inline
def plusone[R][x...] = R[x...] + 1
def foo = (1, 3); (4, 5); (5, 6, 10)
def bar = plusone[foo]
def output = bar[4]
```

## Multiple Arities

Mathematically speaking, an individual relation has a fixed arity.
However, Rel lets you define relations with different arities but the same name.
This is sometimes known as *overloading*, where you can treat a union of relations as a single relation.
For example:

```
// read query
def foo = 1, 2
def foo = 1, 4, 6
def output = foo[1]
```

`foo`

`output`

The built-in predicate `arity`

can be used to check the arity, or arities, of a relation `R`

.
For example:

```
// read query
def output = arity[{6 ; (7, 8) }]
```

Thus, if `count[arity[R]] = 1`

, then `R`

is not overloaded by arity.

## Varargs

To write more general code that works for many different arities, Rel provides a varargs mechanism,
where `x...`

matches zero or more variables. For example, the Standard Library defines `first`

and `last`

as:

```
@inline
def first[R](x) = ∃(y... : R(x, y...))
@inline
def last[R](y) = ∃(x... : R(x..., y))
```

These could also be defined as:

```
@inline
def first[R](x) = R(x, y...) from y...
@inline
def last[R](y) = R(x..., y) from x...
```

As another example, Basic Syntax
discusses how `R[S]`

only works for unary relations `S`

.
You can write a more general version as follows:

```
// read query
@inline
def prefix_restrict[PREFIX, R] = R[x...] from x... in PREFIX
def foo = (1, 2, 3); (4, 5, 6); (7, 8, 9)
def r = {(1, 2); (4, 5)}
def output = prefix_restrict[r, foo]
```

Note that `prefix_restrict`

is closely related to the Standard Library’s
`prefix_join`

, or `<:`

, discussed below.

When using varargs, keep in mind that the system needs to compute the arity of any requested predicates.
For an overloaded relation, this should be a finite number of arities.
For example, the following query succeeds, and `foo(1)`

is `true`

:

```
// read query
@inline def foo(vs..., 1) = true
def output:yes = foo(1)
```

When computing `output`

, the system sees that `foo`

has arity 1, and thus `vs...`

is an empty list of variables.

However, the following query fails; while `foo(1)`

constrains `foo`

to have exactly arity 1,
`foo[1]`

only constrains it to be nonzero, so `foo`

could have any
one of an infinite number of arities:

```
@inline def foo(vs..., 1) = true
def output:yes = foo[1]
```

Note: A definition such as `def output:yes = foo(1)`

is a concise form of `def output[:yes] = foo(1)`

.
This copies the value of `foo(1)`

into the relation `output`

, extending its tuples with a new first element: the Symbol `:yes`

.
In this example the technique is used to make it more explicit that `foo(1)`

evaluates to `true`

, that is, `{()}`

.
More generally, it is very useful for distinguishing between the results of different queries in the same “run.”
See, for instance, the examples in Relational Equality.

### A Word About Arities

In general, using relations with small arities and working with normalized data is preferred. This keeps the arities small and helps with performance, readability, and correctness. Therefore, while varargs are very useful when writing general utilities — see the Standard Library for examples — the arity of individual relations should be as small as possible but, for best performance, the relations should be normalized in Graph Normal Form. Note how even wide CSV tables are ingested as a set of ternary relations, for example.

## Multiple Types

Relations can also be overloaded by *type*, meaning that they can contain tuples with different types.
You can think of each combination of types as a separate relation.
A Rel expression can also refer to a combination of different arities and types.
For example:

```
// read query
def myrel = 1; 2.0
def myrel = ("a", "b") ; (3, 4)
def output = myrel
```

## Specialization: Symbols

Related to overloading by type, relations can be *specialized*, which separates them into different relations
depending on the particular values they take.

This is always the case for Symbols, which are values starting with a colon (`:`

), such as `:a`

and `:b`

below.
For example:

```
// read query
def foo[:a] = 1
def foo[:b] = 2
def output = foo
```

When using the RAI Console Editor in your browser,
you will notice that the “physical” output view shows the specialized relations for `:a`

and for `:b`

separately.

Imported CSV relations are specialized by the column name, which is a symbol, so each column becomes a separate relation.

### Symbols and Base Relation Updates

You can sometimes use the symbol `:relname`

to refer to the relation `relname`

, particularly when updating base relations, using `insert[:relname]=...`

and `delete[:relname]=...`

.
See Working With Base Relations.

### Symbols and Modules

The correspondence between Symbols and specialized relations also shows up in modules,
where `module:foo`

is the same as `module[:foo]`

.
See Modules for more details.

## Recursion

Rel supports recursive definitions, provided the recursion is well-founded (has a base case) and the relations computed are finite.

```
// read query
def fib[0] = 0
def fib[1] = 1
def fib[x in range[2,10,1]] = fib[x-1] + fib[x-2]
def output = fib
```

See Recursion for more details.

For readers familiar with logic programming and Datalog, note that all relations are currently computed in a bottom-up fashion, rather than top-down/on-demand.
Combining negation and recursion is supported, as long as the resulting program is *stratified*,
which means, roughly, that there are no cyclic recursive definitions with a negation in the loop, as in `def p = not p`

.

## Combining Multiple Rules

As mentioned in Basic Syntax, the definitions for any given relation are combined.

This is the case even if the definitions are in separate installed models or libraries. It can be useful to extend existing relations for new types or arities. This applies even to recursive definitions, where you can add new base cases or new recursive definitions to an existing one.

```
// model
def rec[0] = 10
def rec[x] = rec[x-1] + 5, range(1, 4, 1, x)
```

The following definition adds a new base case:

```
// read query
def rec[3] = 200
def output = rec
```

The order of the rules does not matter — and they can be separated by other rules, or even be installed separately.

Multiple rules can always be combined into a single rule that does a big disjunction between the different cases. Separating the cases into different rules is usually more natural and readable.

Another interesting feature of this example is that while the first two rules for `rec`

were *installed*,
the extra base case was part of a query, which enables it only for the effects of that query itself.
This means that if you ask for `rec`

again,
you get only the values from the rules that were originally installed:

```
// read query
def output = rec
```

## Point-Free Syntax

Rel’s syntax supports *point-free* notation, where you can omit argument variables when
their omission does not lead to ambiguity.
Consider the following example:

`def mydomain(x) = range(1, 7, 1, x)`

You can write it instead like this:

`def mydomain = range[1, 7, 1]`

Again, conjunctions correspond to `,`

and disjunctions to `;`

. For example:

`def myrel(x, y) = r1(x) and r2(y)`

This can be expressed point-free as:

`def myrel = r1, r2`

In both cases, you get the cross product of `r1`

and `r2`

.

To see how `;`

corresponds to `or`

:

`def myrel(x, y) = r1(x, y) or r2(x, y)`

This is equivalent to:

`def myrel = r1 ; r2`

As another example, consider this:

```
// read query
def a = {(1, 2)}
def b = {(1, 2) ; (3, 4)}
def myrel(x, y) = a(x, y) and b(x, y)
def output = myrel
```

You can use intersect from the Standard Library and instead write it like this:

`def myrel = intersect[a, b]`

You can even use point-free recursive definitions, such as this one, for the transitive closure of the binary relation `r`

—
see below for an explanation of the composition operator `.`

:

```
// read query
def r = {(1, 2); (2, 3); (3, 4); (2, 5)}
def tcr = r
def tcr = tcr.r // see section below for the meaning of "."
def output = tcr
```

Compared to the “point-wise” alternative where variables are explicitly mentioned, point-free Rel code can be easier to read and faster to write, since it needs fewer variables. It can sometimes be harder to debug, particularly regarding arities, which will not be obvious from reading the code.

Note that writing `def myrel = a and b`

will give an arity error if `a`

and `b`

do not have arity 0, which `and`

expects.
This is also the case for `or`

, `not`

, and `implies`

.

## Relational Equality

In Rel, `=`

means equality between individual, or scalar, values.
That is, when `=`

is used in a formula;
the ”=” used in `def myrel = ... `

has a special status as a reserved symbol.
For example, `3 = 2 + 1`

is `true`

,
and writing `x = y`

supposes that `x`

and `y`

are individual values.

To test equality between relations, use `equal`

:

```
// read query
def output = if equal({1; 2; 3} , {3; 2; 1; 2}) then "yes" else "no" end
```

Since Rel distributes scalar operations over relation values, using `=`

for relations can give confusing results.
For example:

```
// read query
// do not use `=` to check equality of relations!
def output:wrong = if {1} = {1; 2; 3} then "equal" else "not equal" end
// use `equals` for relations:
def output:right = if equal({1}, {1; 2; 3}) then "equal" else "not equal" end
```

Technical footnote: If `R1`

and `R2`

are relations, then `R1 = R2`

if and only if their intersection is nonempty.

Use `=`

to compare individual values, and `equal`

to compare relations.

## Useful Relational Operators

The Rel Standard Library defines many useful relational operations.

This section describes some of the most commonly used relational operations.

### Composition

Relational composition, indicated by a dot (`.`

), is a shortcut for joining the last element of a relation
with the first element of another — usually a foreign key:

```
// read query
def order_products = {(12, 3213) ; (10, 3213) ; (7, 9832)}
def product_names = {(3213, "laptop"); (9832, "iphone"); (45353, "TV")}
def output = order_products.product_names
```

As another example, if `parent(x, y)`

holds when `x`

is a parent of `y`

, then `x : x.parent.parent`

is the “grandparent” relation:

```
// read query
def parent = {("bill", "alice") ; ("alice" , "bob") ;
("alice" , "mary") ; ("jane", "john")}
def output = "bill".parent.parent
```

### Prefix and Suffix Joins

Prefix join, written as `prefix_join[R, S]`

or `R <: S`

,
generalizes `[]`

to get the elements of a relation `S`

that have a prefix in `R`

.
Using varargs notation,
`R <: S`

contains the tuples `(x..., y...)`

in `S`

where `(x...)`

is in `R`

.
For example:

```
// read query
def r = {(1, 2); (2, 5)}
def s = {(1, 2, 3); (1, 5, 7); (1, 2, 8); (2, 5, 9)}
def output = r <: s
```

```
// read query
def json = parse_json["""{"a": {"b": 1, "c": 2}, "d": 3}"""]
def output = :a <: json
```

Suffix join, written as `suffix_join[S, R]`

or `S :> R`

is similar, but matches suffixes.
`R :> S`

contains the tuples `(x..., y...)`

in `R`

where `(y...)`

is in `S`

:

```
// read query
def r = {(1, 2, 3); (1, 5, 7); (1, 2, 8); (2, 5, 9)}
def s = {(2, 3); (5, 9)}
def output = r :> s
```

```
// read query
@inline def even(x) = (x%2=0)
def json = parse_json["""[ {"a": 1, "b": 2}, {"a": 3, "b": 4, "c": 6} ]"""]
def output = json :> even
```

Here, the prefix `([], n)`

indicates a JSON list, with `n`

as the index. For details on how JSON is represented in Rel,
see JSON Import and Export.

### Left and Right Override

The left override operator is usually applied to relations of tuples `(k..., v)`

with a functional dependency from the initial (key) arguments `k...`

to the last argument `v`

(the value).
It lets you merge two such relations, giving precedence to one over the other.

`left_override[R, S]`

, also written as `R <++ S`

,
contains all the tuples `(x..., v)`

of `R`

, plus all the tuples `(x1..., v1)`

of `S`

where `x1...`

is not in `R`

.
Often, `S`

specifies default values for keys that are missing in `R`

.

As a mnemonic, when you read `R <++ S`

you can imagine `S`

injecting new values into `R`

, but only when those keys are missing — so `R`

is overriding `S`

.

```
// read query
def base = ("a", 2) ; ("b", 4)
def defaultvalues = ("a", 10) ; ("c", 20)
def combined = (base <++ defaultvalues)
def output = combined
```

A common use case for override is making explicit default values,
using a constant relation for the default.
You can write `count[R] <++ 0`

if you want the
count of an empty relation to be
`0`

rather than empty. As another example, a counter can be defined using a base relation as follows:

```
// write query
def delete[:counter] = counter
def insert[:counter] = (counter + 1) <++ 0
```

Note that without `delete`

, the consecutive values will accumulate in `counter`

, rather than replace each other.
See Working With Base Relations for more details.

Here is an example of how you can add defaults for a domain,
with a definition using `in`

:

```
// read query
def base = ("a", 10)
def mydomain = ("a" ; "b" ; "c")
def filled[x in mydomain] = base[x] <++ 0
def output = filled
```

Rel also provides `++>`

(right_override, which swaps the order of the arguments.

As a special case, `count[R] <++ 0`

gives us a count of zero for empty sets.
See Aggregating Over Empty Relations.

## Types

Rel provides unary relations for testing the type of primitive values.
These tests include `String`

, `Number`

, `Int`

, and `Float`

,
which take one argument, and counterparts for fixed-width types, such as
`Floating`

. Example: `Floating[32, float[32, 3.0]]`

is `true`

.

See Rel Data Types for details on all of the primitive types.

Types are particularly useful for enforcing schemas as integrity constraints. For example,

`ic { subset(myrel, (Int, Float, String) ) }`

will make sure that `myrel`

has the given type signature (`Int × Float × String`

).

This works for both base relations and derived relations. If `myrel`

is a base relation, then any insert that tries to add something of the wrong type
will fail.
If `myrel`

is a derived relation, then any database update that would cause it to be populated by the wrong type will also fail.

## Summary

This article has covered more advanced features of Rel, including higher-order definitions, overloading by arity and types, varargs, specialization, and point-free syntax.

## See Also

For more on other important concepts in Rel, see:

The Rel Libraries also provide many more useful relational operators and utilities.