Purpose of the REPL

The Miller read-evaluate-print loop is an interactive counterpart to record-processing using the put/filter domain-specific language.

Using put and filter, you can do the following:

Specify input format (e.g. --icsv), output format (e.g. --ojson), etc. using command-line flags.
Specify filenames on the command line.
Define begin {...} blocks which are executed before the first record is read.
Define end {...} blocks which are executed after the last record is read.
Define user-defined functions/subroutines using func and subr.
Specify statements to be executed on each record -- which are anything outside of begin/end/func/subr.
Example: mlr --icsv --ojson put 'begin {print "HELLO"} $z = $x + $y; end {print "GOODBYE"}

Using the REPL, by contrast, you get interactive control over those same steps:

Specify input format (e.g. --icsv), output format (e.g. --ojson), etc. using command-line flags.
Specify filenames either on the command line or via :open at the Miller REPL.
Read records one at a time using :read.
Skip ahead using statements :skip 10 or :skip until NR == 100 or :skip until $status_code != 200.
Similarly, but processing records rather than skipping past them, using :process rather than :skip.
Define begin {...} blocks; invoke them at will using :begin.
Define end {...} blocks; invoke them at will using :end.
Define user-defined functions/subroutines using func/subr; call them from other statements.
Interactively specify statements to be executed on the current record.
Load any of the above from Miller-script files using :load.
Furthermore, any DSL statements other than begin/end/func/subr loaded using :load -- or from multiline input mode which is where you type < on a line by itself, enter the code, then type > on a line by itself -- will be remembered and can be invoked on a given record using :main. In multiline mode and load-from-file, semicolons are required between statements; otherwise they are not needed.

At this REPL prompt you can enter any Miller DSL expression. REPL-only statements (non-DSL statements) start with :, such as :help or :quit. Type :help to see more about your options.

No command-line-history-editing feature is built in but rlwrap mlr repl is a delight. You may need brew install rlwrap, sudo apt-get install rlwrap, etc. depending on your platform.

The input "record" by default is the empty map but you can do things like $x=3, or unset $y, or $* = {"x": 3, "y": 4} to populate it. Or, :open foo.dat followed by :read to populate it from a data file.

Non-assignment expressions, such as 7 or true, operate as filter conditions in the put DSL: they can be used to specify whether a record will or won't be included in the output-record stream. But here in the REPL, they are simply printed to the terminal, e.g. if you type 1+2, you will see 3.

Examples:

$ mlr repl
Miller v6.0.0-dev
Type ':help' for on-line help; ':quit' to quit.
[mlr]

[mlr] 1+2
3

[mlr] x=3  # These are local variables
[mlr] y=4
[mlr] x+y
7

[mlr] <
func f(a,b) {
  return a**b
}
>
[mlr] f(7,5)
16807

[mlr] :open foo.dat
[mlr] :read
[mlr] :context
FILENAME="foo.dat",FILENUM=1,NR=1,FNR=1
[mlr] $*
{
  "a": "eks",
  "b": "wye",
  "i": 4,
  "x": 0.38139939387114097,
  "y": 0.13418874328430463
}
[mlr] f($x,$i)
0.021160211005187134
[mlr] $z = f($x, $i)
[mlr] $*
{
  "a": "eks",
  "b": "wye",
  "i": 4,
  "x": 0.38139939387114097,
  "y": 0.13418874328430463,
  "z": 0.021160211005187134
}

Implementation of the REPL

This is a small modification around the CST and the put verb.. Most of the keystroking here is for online help and command-line parsing.

One subtlety is that non-assignment expressions like NR < 10 are filter statements within put -- they can be used to control whether or not a given record is included in the output stream. Here, in the REPL, these expressions are simply printed to the terminal. And for :skip until ... or :process until ..., they're used as the exit condition to break out of reading input records.

File structure

types.go -- data types including the Repl class
entry.go -- shell command-line entry point to the Miller repl command line. E.g. handles mlr repl --json which is typed at the shell prompt, and starts a command-line session at the Miller REPL prompt.
session.go -- constructs a Repl object and ingests command lines, dispatching them either to the DSL (e.g. $z = $x + $y) or to the non-DSL verb handler (e.g. :open foo.dat or help).
prompt.go -- Handling for default and customized banners/prompts for the Miller REPL.
dsl.go -- Handler for taking DSL statements typed in interactively by the user, parsing them to an AST (via the parsing package), building a CST from the AST, and executing the CST.
verbs.go -- Handlers for non-DSL statements like :open foo.dat or :help.