The new Life of tap()

Background

I’m designing and implementing Next Generation Shell, a programming language (and a shell) for “DevOps” tasks (read: running external commands and data manipulation are frequent).

I came across a programming pattern (let’s call it P) as follows:

1. An object is created
2. Some operations are performed on the object
3. The object is returned from a function (less frequently – stored in a variable)

P Using Plain Approach

The typical code for P looks in NGS like the following:

F my_func() {
  my_obj = MyType()
  my_obj.name = "blah"
  my_obj.my_method(...)
  my_obj  # last expression is evaluated and returned from my_func()
}

The above looks repetitive and not very elegant. Given the frequency of the pattern, I think it deserves some attention.

Attempt 1 – set()

In simpler but pretty common case when only assignment to fields is required after creating the object, one could use set() in NGS:

F my_func() {
  MyType().set(name = "blah")
}

or, for multiple fields:

F my_func() {
  MyType().set(
    name = "blah"
    field2 = 100
    field3 = "you get the idea"
  )
}

Side note: parameters to methods can be separated by commas or new lines, like in the example above.

I feel quite OK with the above but the cons are:

1. Calling a method is not supported (unless that method returns the original object, in which case one could MyType().set(...).my_method())
2. Setting of fields can not be interleaved in a straightforward manner with arbitrary code (for example to calculate the fields’ values)

Attempt 2 – tap()

I’m familiar with tap() from Ruby. It looked quite useful so NGS also had tap() for quite a while. Here is how P would look like in NGS when implemented with tap():

F my_func() {
  MyType().tap({
    A.name = "blah"
    A.my_method()
  })
}

Tap takes an arbitrary value, runs the given callback (passing that value as the only argument) and returns the original value. It is pretty flexible.

Can’t put my finger on what’s exactly is bothering me here but the fact is that I was not using tap() to implement P.

Attempt 3 – expr::{ … }

This one is very similar to tap() but it is syntactically distinct from tap.

F my_func() {
  MyType()::{
    A.name = "blah"
    # arbitrary code here
    A.my_method()
  }
}

I think the main advantage is that P is easily visually distinguishable. For example, if you only want to know the type of the expression returned, you can relatively easy skip everything between ::{ and } . Secondary advantage is that it’s a slightly less cluttered than tap().

Let’s get into the details of how the above works.

Syntax

1. MyType() in our case is an expression. Happens to be a method call which returns a new object.
2. :: – namespace field access operator. Typical use case is my_namespace::my_field.
3. { ... } – anonymous function syntax. Equivalent to a function with three optional parameters (A, B, and C, all default to null).

Note that all three syntax elements above are not unique to this combination. Each one of them is being used in other circumstances too.

Up until recently, the :: syntax was not allowing anonymous function as the second argument. That went against NGS design: all methods should be able to handle as many types of arguments as possible. Certainly limiting arguments’ types syntactically was wrong for NGS.

Semantics

In NGS, any operator is transformed to a method call. :: is no exception. When e1::e2 is encountered, it is translated into a call to method :: with two arguments: e1 and e2.

NGS relies heavily on multiple dispatch. Let’s look at the appropriate definition of the :: method from the standard library:

F '::'(x, f:Fun) {
  f(x)
  x
}

Not surprisingly, the definition above is exactly like the definition of F tap() ... (sans method and parameters naming).

Examples of expr::{ … } from the Standard Library

# 1. Data is an array. Each element is augmented with _Region field.
data = cb(r)::{
  A._Region = ConstIter(r)
}


# 2. push() returns the original object, which is modified in { ... }
F push(s:Set, v) s::{ A.val[v] = true }


# 3. each() returns the original object.
# Since each() in { ... } would return the keys() and not the Set,
# we are working around that with s::{...}
F each(s:Set, cb:Fun) s::{ A.val.keys().each(cb) }


# 4. Return what c_kill() returns unless it's an error
F kill(pid:Int, sig:Int=SIGNALS.TERM) {
  c_kill(pid, sig)::{
    A == -1 throws KillFail("Failed to kill pid $pid with signal $sig")
    A != 0 throws Error("c_kill() did not return 0 or -1")
  }
}

Side note: the comments are for this post, standard library has more meaningful, higher level comments.

A Brother Looking for Use Cases

While changing syntax to allow anonymous function after ::, another change was also made: allow anonymous function after . so that one could write expr.{ my arbitrary code } . The whole expression returns what the arbitrary code returns. Unfortunately, I did not come across (or maybe haven’t noticed) real use cases. The appropriate . method in the standard library is defined as follows:

F .(x, f:Fun) f(x)

# Allows
echo(5.{ A * 2 })  # 10

Have any use cases which look less stupid than the above? Let me know.