Feather in 5 Minutes

If you have been around long enough to remember TCL, or curious enough to have taken a look for no good reason, you should feel right at home.

Feather is TCL in the sense that it chooses the same names and building blocks for the problems solved by TCL. This also makes it a good fit for LLMs, as they can leverage their existing training data.

If you are new: TCL is really simple.

Syntax

# at the start of a command, # introduces a comment
set port 8080

Commands are lists of word. In this example set got the words port and 8080 as arguments, defining the variable port to hold the word 8080.

Commands are separated by spaces, and line breaks separate commands:

set host localhost
puts "Listening on $host:$port"

You can add " around your word to prevent the spaces from breaking it up.

In such double quoted strings, $ substitutes value. The example above prints:

Listenining on localhost:8080

because localhost was assigned to the host variable, and 8080 to the port variable.

To prevent substitutions in words, use braces: {, and }:

puts {Listening on $host:$port}

prints

Listenining on $host:$port

Now for the last important rule: to get the value of another command, you need to use [ and ].

dict set config baseURL "https://www.feather-lang.dev"
puts [dict get $config]/in-5-minutes.html

# Output: https://www.feather-lang.dev/in-5-minutes.html

On Values and Types

Feather, just like TCL, does not assign a type to a value.

Individual commands are free to interpret words however they wish.

When Feather needs to treat a value as a list, number, or dictionary it performs the conversion once and remembers the representation, until the same value is interpreted again as something else.

This allows Feather to save work, while still staying malleable like clay:

if {$port < 1024} {
  error "Reserved port"
}

The command if is a regular command that decides to treat its first word as an expression ($port < 1024 here), and its second word as a script to execute in case the expression evaluated to 1.

It is not special syntax.

In fact, you can easily define your own language building blocks using uplevel, which evaluates its argument as a script in another context:

# to get this syntax
set such Feather
set much wow
repeat 3 { puts "Such $such. Much $much." }

# we only need to define this
proc repeat {times script} {
  for {set i 0} {$i < $times} {incr i} {
    uplevel 1 $script
  }
}

Here we interpret whatever is held in script as code to run, and run it in the context where repeat is called.

Now, all that's left is learning about Feather's built-in commands.