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Elektra
0.8.24
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A plugin that type checks specifications before setting keys and after getting keys from a mounted specification.
The type checker uses regular expressions as the foundation. It assigns a regex to each key describing its possible contents, thus defining a key's type. Links between keys will fail the type check if the regexes describing the linked keys are not compatible with each other, i.e. the regex of the linked key is equal or a subset of the regex of the linking key. This concept is not only useful for defining links. It can be used to restrict the input types of transformations for instance as well.
There are basically two ways to use type checker plugin. First we describe how the plugin is used in general for users. Afterwards we describe how the type checker plugin can be extended with the semantics of additional configuration specification keywords. This is interesting for plugin developers who want to make use of the type system for their work as well.
In order to use the type checker, mount a configuration specification along with this plugin. There are already a lot of commonly used keywords supported out of the box specified in the file prelude.ini. Usually one wants to mount that file as well beforehand for the standard configuration.
kdb mount prelude.ini spec/<path>/spec/elektra ini kdb mount <specification> spec/<path> <storage plugin to read the specification> typechecker
Retrieving any key from the specification using kdb get <path> will cause the type checking to happen. The type checker will issue a warning if it detects any problems with it. In case a key covered by a configuration specification is accessed programmatically, the type checking will also happen if the type checker is mounted.
When altering a mounted specification using kdb set <path> or kdb setmeta <path> <value>, the type checker will issue an error if the newly added key or metakey will lead to an inconsistent configuration specification according to the type system specification.
It is often necessary to set multiple specification keywords at once in order to transform a valid configuration specification to another valid one. Therefore some kind of transaction is required so the type checking only happens when the whole changes have been made. The easiest way to do this is to use the command kdb shell. An example of its usage is shown below. In case you prefer a graphical tool to edit configuration specifications you can use the qt-gui for Elektra.
Please keep in mind that its better to unmount in the reverse way, so first unmount your specification and then prelude.
In case a new plugin has been written which can be described using regular expressions, a plugin developer may want to teach the type system about this plugin. In order to do that, each configuration specification may contain a section describing the effects of keywords. There are three metakeys that are interpreted by the typechecker, elektra/spec/order, elektra/spec/type and elektra/spec/impl. These metakeys get explained and introduced along the following examples. Let's take the example of the fallback keyword, that would be formalized in the ini format as follows:
In the implementation elektra/spec/impl we are limited to the two functions link and intersect forming the basic building blocks for describing more complex keywords. They respective type signatures are RegexContains b a => Key b :: . -> Key a -> Key a for link and Intersectable (RegexIntersection a b) => Key a -> Regex b -> Key (RegexIntersection a b) for intersect. A type signature, inspired by Haskell's type signatures, typically has two parts. The first part describes its constraints followed by =>. Then its parameter's types follow separated by a ->, where the last parameter type is the result type. A constraint can bei either a containment constraint or a intersection constraint, written RegexContains or Intersectable followed by two type variables. Type variables may as well be a string constant describing a regex as we will see in the next example. In our case RegexContains b a means that the regex stored in the type variable a is either equal to or a subset of the regex stored in the type variable b. The type variables arise from the type parameters. A type parameter may either refer to a Key or to a Regex. A Key may either refer to the path of another key which is given relatively to the current key after the :: symbol, or to the annotated key itself, in which case no extra annotation is necessary. A Regex refers directly to a regex which can then be used as a type variable. Last, we see the type RegexIntersection that represents the result of the intersection of two regexes. The Intersectable constraints checks whether such an intersection is possible. This is used to prevent two incompatible checks from being applied to a single key. For instance, consider a check for letters and another check for numbers, there would be nothing that could be stored in such a key. The metakey elektra/spec/order is used to specify the order in which checks or links are applied. This is interesting because links should happen when all the checks on a single key have already been applied so all the limitations of a key are already in effect.
Summing up the above example can be read as "If the regex of the key b, which path is obtained by taking the value of the current key, relatively referred to via . is equal to or a subset of the regex describing the current key, this link is allowed. In that case the type of the current key will not be changed and remain to be the regex stored in the type variable a.
Let us look at another example which makes use of intersect. Suppose you want to make a specification keyword that checks whether the key's content lies with the boundaries of a 16 bit unsigned short value, corresponding to the range 0-65535, or rewritten as a regex, ([0-9]|[1-8][0-9]|9[0-9]|[1-8][0-9]{2}|9[0-8][0-9]|99[0-9]|[1-8][0-9]{3}|9[0-8][0-9]{2}|99[0-8][0-9]|999[0-9]|[1-5][0-9]{4}|6[0-4][0-9]{3}|65[0-4][0-9]{2}|655[0-2][0-9]|6553[0-5])
This function means "If the regex represented by the type variable a, which in that case refers to the current key's regex, can be intersected with the regex depicting the range, the check passes successfully and the resulting type is the intersection between the two regexes." This approach is typically used with checker plugins which further restrict what kind of contents a key may contain, and each intersection limits the type more and more. The constraints prevents that two incompatible checks can be used for a single key.
It is also possible to explicitly state the type signature for a metakey in case its effect cannot be specified and inferred using link or intersect. In that case we can use undefined as the result of the function. For instance the following example defines a transformation that can be used on keys representing letters, and outputs a number:
Create a sample configuration specification with three keys. As it is valid, there will be no error issued. We rely on the existence of prelude.ini, which already contains the type definitions for check/range, check/long and fallback/# and mount it along. Please note not to mount prelude below the specification. We use kdb shell to delay the type checking until we have finished writing the whole specification.
Add an invalid link now and see how it refuses the specification, showing the erroneous parts instead. As the two keys represent the ranges 0-5000 and 7200-10000, they obviously cannot be linked together.
As indicated in the limitations, error messages are currently rather raw and haskell-focused:
It is still fairly easy to map the error message back to the configuration specification. The above error message shows that there was an error when type checking the specification of key2 as indicated by the line "In an equation for ...". The line "In the expression: ..." then shows the exact part where its happen. The incompatibility arises during checking the fallback from key2 to key1. The regexes describing the contents of the two keys are not compatible with each other. The first key contains numbers between 0 and 5000, while the secondkey is restricted to numbers between 7200 and 10000.
This test specification has no errors by default and will thus report nothing, but if you alter it you can experiment with the type checker. If Elektra is compiled with the ENABLE_LOGGER flag, it will log the inferred types in all cases so the type behavior can be observed when getting/setting a key in a specification.
Furthermore the following haskell dependencies need to be installed to the sandbox as explained in the bindings readme:
1.8.13