A type class for monads which implement (or mock) low-level Linux sysfs GPIO operations.

The sysfs pin handle type. Currently it's just a newtype wrapper around a Pin. The constructor is exported for convenience, but note that the implementation may change in future versions of the package.

An instance of MonadGpio which translates actions in that monad to operations on Linux's native sysfs GPIO interface.

Test whether the sysfs GPIO filesystem is available.

Return a list of all pins that are exposed via the sysfs GPIO filesystem. Note that the returned list may omit some pins that are available on the host but which, for various reasons, are not exposed via the sysfs GPIO filesystem.

Test whether the pin is already exported.

Export the given pin.

Note that, if the pin is already exported, this is not an error; in this situation, the pin remains exported and its state unchanged.

Export the given pin.

Note that, unlike exportPin, it's an error to call this action to export a pin that's already been exported. This is the standard Linux sysfs GPIO behavior.

Unexport the given pin.

Note that, if the pin is already unexported or cannot be unexported, this is not an error. In this situation, the pin remains exported and its state unchanged.

Unexport the given pin.

Note that, unlike unexportPin, it is an error to call this action if the pin is not currently exported. This is the standard Linux sysfs GPIO behavior.

Test whether the pin's direction can be set via the sysfs GPIO filesystem. (Some pins have a hard-wired direction, in which case their direction must be determined by some other mechanism, as the direction attribute does not exist for such pins.)

Read the pin's direction.

It is an error to call this action if the pin has no direction attribute.

Set the pin's direction.

It is an error to call this action if the pin has no direction attribute.

Note that, in Linux sysfs GPIO, changing a pin's direction to out will also set its physical signal level to low.

NB: in Linux sysfs, if an input pin is cofigured for edge- or level-triggered reads, it's an error to set its direction to out. However, this action will handle that case gracefully by setting the pin's edge attribute to none before setting the pin's direction to out.

Pins whose direction can be set may be configured for output by writing a PinValue to their direction attribute, such that the given value will be driven on the pin as soon as it's configured for output. This enables glitch-free output configuration, assuming the pin is currently configured for input, or some kind of tri-stated or floating high-impedance mode.

It is an error to call this action if the pin has no direction attribute.

NB: for some unfathomable reason, writing high or low to a pin's direction attribute sets its physical signal level; i.e., it ignores the value of the pin's active_low attribute. Contrast this behavior with the behavior of writing to the pin's value attribute, which respects the value of the pin's active_low attribute and sets the pin's logical signal level.

Rather than slavishly following the Linux sysfs GPIO spec, we choose to be consistent by taking into account the pin's active level when writing the direction attribute. In other words, the PinValue argument to this action is the logical signal level that will be set on the pin. If you're using this action to program directly to the Linux sysfs GPIO interface and expecting things to behave as they do with raw sysfs GPIO operations, keep this in mind!

Read the pin's signal level.

Note that this action never blocks, regardless of the pin's edge attribute setting.

A blocking version of readPinValue. The current thread will block until an event occurs on the pin as specified by the pin's current edge attribute setting.

If the pin has no edge attribute, then this action's behavior is undefined. (Most likely, it will block indefinitely.)

Same as pollPinValue, except that a timeout value, specified in microseconds, is provided. If no event occurs before the timeout expires, this action returns Nothing; otherwise, it returns the pin's value wrapped in a Just.

If the timeout value is negative, this action behaves just like pollPinValue.

When specifying a timeout value, be careful not to exceed maxBound.

If the pin has no edge attribute, then this action's behavior is undefined. (Most likely, it will time out after the specified delay and return Nothing.)

NB: the curent implementation of this action limits the timeout precision to 1 millisecond, rather than 1 microsecond as the timeout parameter implies.

Set the pin's signal level.

It is an error to call this action if the pin is configured as an input pin.

Test whether the pin has an edge attribute, i.e., whether it can be configured for edge- or level-triggered interrupts.

Read the pin's edge attribute.

It is an error to call this action when the pin has no edge attribute.

Write the pin's edge attribute.

It is an error to call this action when the pin has no edge attribute, or when the pin is configured for output.

Read the pin's active_low attribute.

Write the pin's active_low attribute.