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Execution Context

Prolog module for reading execution context variables. Enables to retrieve execution specific variables from command line options, environment variables, configuration file or fallback to some predefined default value.

USAGE

Use module execution_context and declare your context variables by invoking context_variable/3 predicate. After that you can use the predicate context_variable/2 or option_or_context_variable/2 predicates to read value of the context variable.

Example:

:- use_module(library(execution_context)).

:- context_variable(my_variable, atom, [long('cli-arg-name'), short(m), env(MY_VARIABLE), default(some_value)]).

hallo :-
    context_variable_value(my_variable, Value),
    writeln(Value).

Details

The variable names are module scoped. The resolution is done in the following order:

  • options if the option_or_context_variable/2 is used
  • command line arguments
  • environment variables
  • config.user.env file
  • config.dev.env file
  • config.env file
  • default value if specified durring declaration.

The values are resolved only once, then cached in local database

Exported predicates from the module execution_context

context_variable(+Name:atom, +Type, +Options:list) is det.

Declares new contextual variable with the module specific name. Repeated declarations for the same Name are possible, the last call is taken into account but not after the value was queried (which would be served from the cache. Type can be one of the atom, bool, number, string, list, list(atom), list(number). in case of list types, the list can be specified as a list of atoms or numbers separate by comma (,) or semicolon, and optionally enclosed into parenthesis or brackets (e.g. [ a, b,c, e], or (1;2; 3 ; 4), or o, p ; q, r ) will work. Elements in the list are trimmed of left and right spaces.

Options can be one of:

  • describe(Message) - provides human readable information about the command that is printed when the CLI help is invoked (for cli options).
  • long(Long) - specifies the long name of the command line parameter prefixed with two dashes. If not provided then the Long name is assumed to be based on the name with characters ' ', '_', ':' replaced by the dash. Value of the variable can be either concatenated into argument with '=' character, or separated by space. For example if the long(some_option) is given then the argument --some-option=value, or --some-option value will both assign value to context variable of the Name.
  • short(Short) - specifies short variant of the command line argument prefixed by '-'. Value must be provided after the space. For example if the option short(s) is given then the argument -s value will assign 'value' to context variable of the Name
  • is_flag(Bool) - if Bool is true, then the system recognize the options with out value and --no-long options, where long correspnds to the long(Atom) option value. Variables of type bool are implicitly considered to be flags
  • env(EnvName) - specifies the name of the environment variable to check for the value of the context variable. If not provided then the EnvName name is assumed to be based on the name with characters ' ', '_', ':' replaced by the undescore.
  • default(Value) - Specifies the default values if no other means provides a value. If the Value is of the form context(Variable), then context variable Variable is evaluated for the default value. Infinite dependency cycles are not resolved, so be carefull here.
  • cli_command(Commands) - Commands is list of CLI commands (see execute_cli/1), for which the variable is intended. This information is only used to describe usage of the command. Either option short or long˙, and option describe˙ must be provided to provide any help information about the variable. The option cli_command([]) is interpreted as the option for all CLI commands.

context_variable_value(+Variable, -Value) is semidet

Unifies Value with the contextual variable. The Variable must be declared using context_variable/3 call. To resolve the contextual variable following steps are executed. The first step that succed will determine the Value

  • Check for short or long variant of command line arguments name associated with the variable
  • Check for the environment variable name
  • Check if the Variable name or environment variable name is assigned in the config.user.env file
  • Check if the Variable name or environment variable name is assigned in the config.dev.env file
  • Check if the Variable name or environment variable name is assigned in the config.env file
  • Check if the default value is provided.

The config.env, config.dev.env, and config.env files are looked up in the file search paths defined as params(config.dev). The params path resolves to current working directory or into the config directory of the current working directory. The name variants are provided by the options of the predicate context_variable/2 or derived from the context variable name.

option_or_context_variable(Option, Options) is semidet

As option/2, but if that fails then it behaves as context_variable(OptionFunctor, OptionArg). Options are checked for the module specific or local name of the option.

Exported predicates from the module execution_cli

execute_cli is det

Same as current_prolog_flag(argv, Arguments), execute_cli(Arguments)

execute_cli(+Arguments:list(atom)) is det

Interprets Arguments which are assumed to be unified by current_prolog_flag(argv, Arguments) call and calls goal registered by register:cli_command/2 directive. The name of the command is determined by the first positional argument. Options are removed from the Arguments list by checking against registered context variables - see execution_context:context_variable/3 predicate.

Throws cli_option(Type, OptionName) exception if required arguments are not provided (see register_cli_command/3), or when the option that cannot be associated with any context variable is found on command line arguments. The exception can be used for print_message call.

register_cli_command(+Command:atom, :Goal, +ArgumentsSpec:list) is det

Registers new command for CLI. The Goal will be invoked when the first positional argument unifies with Command. The Goal is invoked as call(Goal, PositionalArguments, Options) where PositionalArguments are all arguments that are not an command line option, and Options maps some of the context variables to the goal options.

ArgumentSpec can contain some of the following terms:

  • describe(Message) - provides human readable information about the command that is printed when the CLI command help or command help is invoked.
  • context(ContextVariable, OptionName) - if call to context_variable_value(ContextVariable, Value) succeeds, then Goal second argument will contain option named OptionName with the value Value. If the context_variable is not resolved then the exception is raised and user informed that the required option is missing
  • optional(context(ContextVariable, OptionName)) - similar to above, but the context variable is not required to be resolved
  • positional(Nth1, OptionName) - associates Nth1 element of positional arguments with option OptionName when calling Goal. If there are not enough positional argumentsthen the exception is raised and user is informed that argument is missing
  • optional(positional(Nth1, OptionName)) - similar to above, but the positional argument is not required to be resolved

Testing

The tests requires some command line options and environment variables, also the availability of config files. The script run-tests.ps1 prepares the environment.

Development

To debug the module, load the debug.pl file into prolog top.