Availability:built-in

`:Condition`

***->**

`:Action ; :Else`

`Condition`succeeds at least once, the semantics is the same as (

`Condition`,

`Action`). If

`Condition`does not succeed, the semantics is that of (

`\+`

`Condition`,

`Else`). In other words, if

`Condition`succeeds at least once, simply behave as the conjunction of

`Condition`and

`Action`, otherwise execute

`Else`. The construct is known under the name if/3 in some other Prolog implementations.

The construct `A` `*->`

`B`, i.e.,
without an
`Else` branch, is translated as the normal conjunction `A`,
`B`.^{bugThe decompiler
implemented by clause/2
returns this construct as a normal conjunction too.}

This construct is rarely used. An example use case is the implementation of optional in sparql. The optional construct should preserve all solutions if the argument succeeds as least once but still succeed otherwise. This is implemented as below.

optional(Goal) :- ( Goal *-> true ; true ).

Now calling e.g., `optional(member(X, [a,b]))`

has the
solutions
`X=a` and `X=b`, while `optional(member(X,[]))`

succeeds without binding `X`.