.. index:: single: validation .. _validation/0:

.. rst-class:: right

object

validation

Constructors for validation terms. A validation term is either valid(Value) or invalid(Errors) where Errors is a list of errors. Validation terms allow applicative-style error accumulation.

| Availability: | logtalk_load(validations(loader))

| Author: Paulo Moura | Version: 1:0:0 | Date: 2026-02-22

| Compilation flags: | static, context_switching_calls

| Provides: | :ref:`type::type/1 <type/0::type/1>` | :ref:`type::check/2 <type/0::check/2>` | Uses: | :ref:`list <list/0>` | :ref:`type <type/0>`

| Remarks:

• Type-checking support: Defines a validation type for use with the type library object.

| Inherited public predicates: | (none)

.. contents:: :local: :backlinks: top

Public predicates

.. index:: of_valid/2 .. _validation/0::of_valid/2:

of_valid/2 ^^^^^^^^^^^^^^

Constructs a validation term holding a valid value.

| Compilation flags: | static

| Template: | of_valid(Value,Validation) | Mode and number of proofs: | of_valid(@term,--nonvar) - one

---

.. index:: of_invalid/2 .. _validation/0::of_invalid/2:

of_invalid/2 ^^^^^^^^^^^^^^^^

Constructs a validation term holding a single error.

| Compilation flags: | static

| Template: | of_invalid(Error,Validation) | Mode and number of proofs: | of_invalid(@term,--nonvar) - one

---

.. index:: of_invalids/2 .. _validation/0::of_invalids/2:

of_invalids/2 ^^^^^^^^^^^^^^^^^

Constructs a validation term holding a list of errors.

| Compilation flags: | static

| Template: | of_invalids(Errors,Validation) | Mode and number of proofs: | of_invalids(@list,--nonvar) - one

---

.. index:: from_goal/4 .. _validation/0::from_goal/4:

from_goal/4 ^^^^^^^^^^^^^^^

Constructs a validation term holding a value bound by calling the given goal. Otherwise returns a validation term with a single error represented by the Error argument.

| Compilation flags: | static

| Template: | from_goal(Goal,Value,Error,Validation) | Meta-predicate template: | from_goal(0,*,*,*) | Mode and number of proofs: | from_goal(+callable,--term,@term,--nonvar) - one

---

.. index:: from_goal/3 .. _validation/0::from_goal/3:

from_goal/3 ^^^^^^^^^^^^^^^

Constructs a validation term holding a value bound by calling the given goal. Otherwise returns a validation term with a single error being the goal error or the atom fail representing goal failure.

| Compilation flags: | static

| Template: | from_goal(Goal,Value,Validation) | Meta-predicate template: | from_goal(0,*,*) | Mode and number of proofs: | from_goal(+callable,--term,--nonvar) - one

---

.. index:: from_goal/2 .. _validation/0::from_goal/2:

from_goal/2 ^^^^^^^^^^^^^^^

Constructs a validation term holding a value bound by calling the given closure. Otherwise returns a validation term with a single error being the closure error or the atom fail representing closure failure.

| Compilation flags: | static

| Template: | from_goal(Closure,Validation) | Meta-predicate template: | from_goal(1,*) | Mode and number of proofs: | from_goal(+callable,--nonvar) - one

---

.. index:: from_generator/4 .. _validation/0::from_generator/4:

from_generator/4 ^^^^^^^^^^^^^^^^^^^^

Constructs validation terms with the values generated by calling the given goal. On goal error or failure, returns a validation term with a single error represented by the Error argument.

| Compilation flags: | static

| Template: | from_generator(Goal,Value,Error,Validation) | Meta-predicate template: | from_generator(0,*,*,*) | Mode and number of proofs: | from_generator(+callable,--term,@term,--nonvar) - one_or_more

---

.. index:: from_generator/3 .. _validation/0::from_generator/3:

from_generator/3 ^^^^^^^^^^^^^^^^^^^^

Constructs validation terms with the values generated by calling the given goal. On goal error or failure, returns a validation term with, respectively, a single error being the goal error or the atom fail representing goal failure.

| Compilation flags: | static

| Template: | from_generator(Goal,Value,Validation) | Meta-predicate template: | from_generator(0,*,*) | Mode and number of proofs: | from_generator(+callable,--term,--nonvar) - one_or_more

---

.. index:: from_generator/2 .. _validation/0::from_generator/2:

from_generator/2 ^^^^^^^^^^^^^^^^^^^^

Constructs validation terms with the values generated by calling the given closure. On closure error or failure, returns a validation term with, respectively, a single error being the closure error or the atom fail representing closure failure.

| Compilation flags: | static

| Template: | from_generator(Closure,Validation) | Meta-predicate template: | from_generator(1,*) | Mode and number of proofs: | from_generator(+callable,--nonvar) - one_or_more

---

.. index:: from_optional/3 .. _validation/0::from_optional/3:

from_optional/3 ^^^^^^^^^^^^^^^^^^^

Converts an optional term to a validation term. Returns a valid term holding the value if the optional term is not empty. Returns an invalid term with the given error otherwise.

| Compilation flags: | static

| Template: | from_optional(Optional,Error,Validation) | Mode and number of proofs: | from_optional(+nonvar,@term,--nonvar) - one

---

.. index:: from_expected/2 .. _validation/0::from_expected/2:

from_expected/2 ^^^^^^^^^^^^^^^^^^^

Converts an expected term to a validation term. Returns a valid term holding the value if the expected term holds a value. Returns an invalid term with the expected term error otherwise.

| Compilation flags: | static

| Template: | from_expected(Expected,Validation) | Mode and number of proofs: | from_expected(+nonvar,--nonvar) - one

---

Protected predicates

(no local declarations; see entity ancestors if any)

Private predicates

(no local declarations; see entity ancestors if any)

Operators

(none)

.. seealso::

:ref:`validation(Validation) <validation/1>`, :ref:`validated <validated/0>`, :ref:`type <type/0>`, :ref:`arbitrary <arbitrary/0>`