structor

Uses EL in Go struct tags to compute struct fields.

In a nutshell

Basic idea is to use simple expression language (EL) within Go struct tags to compute struct fields based on other fields or provided additional context.

It can be viewed/used as an advanced struct builder or constructor. Hence the name. Another possible use case is a "struct walker" or "struct field visitor".

Implementation uses reflection and EL interpretation, so, it is relatively slow by design, and, because of that, should be used for infrequent tasks. For example, during program initialization. Or in cases when all other alternatives are equally slow.

Initial implementation uses "text/template" as EL, it get fields name, value, tags, full struct and extra context as a "dot" context and either use special "set" custom function to save new field's value as object or just stores result of the template evaluation as string into the field.

Map of the custom functions can be passed to EL engine for use in the expressions. "set" is a special custom EL function which stores argument into the currently processed field. Special case: if annotated field is struct and result can not be converted to it, then result stored into .Sub, and evaluation executed recursively on the inner struct.

Another special custom EL function is "eval". This function allows to invoke interpreter from within EL expression to evaluate another expression. Example usage is when expression of interest come from another field or extra context. For instance, it can come from configuration file.

Simple namespaces and "import" mechanism provided for custom functions - whole map of custom functions can be combined from several maps with (optional) custom prefixes, prepended to every function name within map. Maps for provided functions placed into separate Go packages and can be imported as needed. When used with Glide, they can be imported as subpackages, allowing to optimize dependencies.

It's simpler to illustrate on examples, see examples in tests (funcs_test.go, goel_funcs_test.go) and Godoc.

Possible use cases

• take a struct, filled up from configuration file using github.com/spf13/viper and compute additional fields:

  • replace referencies in settings to their values (for example base URL in othre URLs); also: templates in the settings themeselves, not in the tags of structure;
  • load short text files into string fields (with file names from config);
  • decode passwords;
  • extract data from environment variables, listed in config;
  • execute bash scripts to compute fields (iconv, openssl, etc);
  • parse complex types from string representation;

• use engines like regexp, xpath or goquery to extract pieces of data from text, xml, html etc. formats into fields (scraping data from html pages, text files or emails); for long and complex expressions it can be convenient to use multiline tags and process whole tag value as single expression;

• registering a custom EL "interpreter", which executes arbitrary custom logic in its Execute() method, it is possible to use structor as a "struct walker" or "field visitor", which will traverse structure fields and invoke custom function for every field, containing custom tag; it can be convenient with ability to use multiline tags;

Ideas of functions, available in expressions

• atoi
• base64/unbase64
• exec - invoke external process (shell, for instance)
• encrypt/decrypt
• env
• eval
• fields
• readFile - read file content to []byte or string
• goquery
• hex/unhex
• match
• math (basic arithmetics)
• replace
• rot13
• set
• split
• standard for package "text/template"
• trimSpace
• xpath
• ... (custom)

Other ideas

• go expressions (using "github.com/apaxa-go/eval")

Godoc

Godoc