Conditionals¶

This page documents the conditional expressions available in the constraint handler. All conditionals can be used just like normal operators known from the Base Types or Collections documentation pages.

Notation¶

On this page, we will expand the notation introduced in the Collections documentation page to also include conditionals.

Even though a Condition is an Expression that evaluates to bool, we will mark it as C in the operator signatures to better indicate that this is the condition the respective conditional is based on.

If (If-Then)¶

Sometimes operations should only be performed when a certain condition is met. For this, the if operator can be used.

Operator	Name	Signature	Description
`if`	If-Then	(C, A) \(\to\) none \| A	If the condition `C` holds, then the Expression `A` is evaluated. Otherwise, the conditional evaluates to none.

Example

variable_define(z, operation(if, (val(bool, true),(val(int,2),())))).

Here z will be assigned the value 2, since the condition is true.

The model will contain the atom:

value(z,val(int,2))

However:

variable_define(z, operation(if, (val(bool, false),(val(int,2),())))).

Here, since the condition is false, z will be assigned none.

The model will contain the atom:

value(z,val(none, none))

Ite (If-Then-Else)¶

The ite operator expands on the if operator by allowing to specify an alternative Expression to be evaluated when the condition does not hold.

Operator	Name	Signature	Description
`ite`	If-Then-Else	(C, A, B) \(\to\) A \| B	If the condition [`C`] holds, evaluates the expression `A`, otherwise the expression `B`.

Example

variable_define(z, operation(ite, (val(bool, true),(val(int,2),(val(int,5),()))))).

Here, just like in the if case, z will be assigned the value 2, since the condition is true.

The model will contain the atom:

value(z,val(int,2))

However:

variable_define(z, operation(ite, (val(bool, false),(val(int,2),(val(int,5),()))))).

This time, when the condition is false, z will be assigned the value 5.

The model will contain the atom:

value(z,val(int,5))

Default¶

The default operator is used to provide a fallback value if the first Expression is undefined (e.g. evaluates to [none]).

Operator	Name	Signature	Description
`default`	Default	(A	none, B) \(\to\) A \| B

Example

variable_define(z, operation(default, (val(int, 2),(val(int,5),())))).

Here, because the value is defined, z will be assigned the value 2.

The model will contain the atom:

value(z,val(int,2))

However:

variable_define(z, operation(default, (val(none, none),(val(int,5),())))).

Here, since the first value is none, z will be assigned the value 5.

The model will contain the atom:

value(z,val(int,5))

hasValue¶

The hasValue operator checks whether an Expression is defined (not none).

Operator	Name	Signature	Description
`hasValue`	Has Value	(T) \(\to\) bool	Returns `true` if the argument is defined, otherwise `false`.

Example

variable_define(z, operation(hasValue, (val(none, none),()))).

Here, since the value is none, z will be assigned the value false.