context-connect

Helper to connect a context to composed functions.

Function composition

Function composition is stringing together a series of functions, where the result of calling one function is passed on to the next function.

Creating chains, or pipelines, of functions is a nice way to assemble functionality from small, focused functions. Often, however, function composition takes f and g and returns a function that first calls g and then passes the result to f. I find it much more intuitive for the order in which the functions are called to be the same as we read them, left-to-right. A couple of useful tools for doing that are:

• lodash flow
• functional-pipeline

Using a context

When chaining a series of functions, you might want to use a context object to gather all the information from the functions. Each function takes the context as a parameter and returns it as a result, having augmented it with the result of its operation.

Using context-connect

The problem with using a context is that some of the functions that you want to use in your pipeline may not expect a context as a parameter, and/or return a context as a result. Rewriting such functions, or wrapping them, is tedious. This is where context-connect can help.

Say you have a pipeline of the following functions:

f1 f2 g f4

The f1, f2, and f4 functions all expect and return the context. The g function, however, accepts two parameters and returns a simple result. How can you connect g into the pipeline?

When you require("context-connect"), you get a function that accepts a series of properties to take out of the context, the function that you want to call with those values, and the property to use when putting the result back into the context.

Continuing with the previous example, here is how you can assemble the functions:

var ctx = require("context-connect");
var _ = require("lodash");

_.flow(f1, f2, ctx("a", "b", g, "c"), f4);

Given that f2 returns context, g would be called with context.a and context.b as parameters. The result would be set as context.c, and context would be passed on to f4.

If the function already accepts a context but returns a simple value instead of the context, you can omit the properties to take out of the context:

_.flow(f1, f2, ctx(g, "c"), f4);

In that case, the context is passed to g and the result is set as context.c.

Similarly, if you want to take properties out of the context to pass to the function, but want to use the result as is, you can omit the result property.

_.flow(f1, f2, ctx("a", "b", g), f4);

This passes context.a and context.b to g, and the result is passed on directly to f4.

License

Copyright © 2015 Fred Daoud

Distributed under The MIT License (MIT).

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.