# `stdlib_linal_state`

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```

## Introduction

The `stdlib_linalg_state` module provides a derived type holding information on the state of linear algebra operations, and procedures for expert control of linear algebra workflows. 
All linear algebra procedures are engineered to support returning an optional `linalg_state_type` variable to hold such information, as a form of expert API. If the user does not require state information but fatal errors are encountered during the execution of linear algebra routines, the program will undergo a hard stop.
Instead, if the state argument is present, the program will never stop but will return detailed error information into the state handler. 

## Derived types provided

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### The `linalg_state_type` derived type

The `linalg_state_type` is an extension of the `state_type` derived type, containing an integer error flag and fixed-size character strings to store an error message and the location of the error state change. 
Fixed-size string storage was chosen to facilitate the compiler's memory allocation and ultimately ensure maximum computational performance.  

A similarly named generic interface, `linalg_state_type`, is provided to allow the developer to create diagnostic messages and raise error flags easily. The call starts with an error flag or the location of the event and is followed by an arbitrary list of `integer`, `real`, `complex`, or `character` variables. Numeric variables may be provided as either scalars or rank-1 (array) inputs. 

#### Type-bound procedures

The following convenience type-bound procedures are inherited from `state_type` and available:
- `print()` returns an allocatable character string containing state location, message, and error flag; 
- `print_message()` returns an allocatable character string containing the state message; 
- `ok()` returns a `logical` flag that is `.true.` in case of successful state (`flag==LINALG_SUCCESS`);
- `error()` returns a `logical` flag that is `.true.` in case of an error state (`flag/=LINALG_SUCCESS`).

#### Status

Experimental

#### Example

```{literalinclude} ../../example/linalg/example_state1.f90
:language: fortran
```

## Error flags provided

The module provides the following state flags, mapped to the general `state_type` error flags: 
- `LINALG_SUCCESS`: Successful execution (equivalent to `STDLIB_SUCCESS`)
- `LINALG_VALUE_ERROR`: Numerical errors (such as infinity, not-a-number, range bounds) are encountered (equivalent to `STDLIB_VALUE_ERROR`).
- `LINALG_ERROR`: Linear Algebra errors are encountered, such as non-converging iterations, and impossible operations (equivalent to `STDLIB_LINALG_ERROR`).
- `LINALG_INTERNAL_ERROR`: Provided as a developer safeguard for internal errors that should never occur (equivalent to `STDLIB_INTERNAL_ERROR`).

## Comparison operators provided

The module provides overloaded comparison operators for all comparisons of a `linalg_state_type` variable with an integer error flag: `<`, `<=`, `==`, `>=`, `>`, `/=`.