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YSH Language Influences

Almost all syntax in YSH comes from another language. This doc lists some of these influences.

Reading this page isn't essential for all users, but it may help some users remember the syntax.

General Philosophy

At a high level, YSH is a bash-compatible shell language that adds features from popular dynamic languages.

Its design is more conservative than that of other alternative shells. Our goals are to:

• Preserve what works best about shell: processes, pipelines, and files.
• Clean up the sharp edges like quoting, ad hoc parsing and splitting
• Integrate features from Python, JavaScript, Ruby, and other languages listed below.

Major Influences

POSIX Shell

The command and word syntax comes from shell:

ls | wc -l                        # pipeline
echo $var "${var} $(hostname)"    # variable and command sub
echo one; echo two                # sequence of commands
test -d /tmp && test -d /tmp/foo  # builtins and operators

Shell-like extensions in YSH:

echo $[42 + a[i]]                 # Expression substitution
cd /tmp { echo hi }               # Block arguments

bash and ksh

We implement many bash semantics, like "named references" for out variables:

f() {
  local -n out=$1    # -n for named reference
  out=bar
}

x=foo
f x
echo x=$x            # => x=bar

Though we discourage dynamic scope. YSH provides a better mechanism called value.Place.

proc f(; out) {
  call out->setValue('bar')
}

var x = 'foo'
f (&x)               # pass a place
echo x=$x            # => x=bar

Python

The YSH expression language is mostly Python compatible. Expressions occur on the right-hand side of =:

var a = 42 + a[i]
var b = fib(10)
var c = 'yes' if mybool else 'no'

Proc signatures take influence from Python:

proc mycopy(src, dest='/tmp') {  # Python-like default value
  cp --verbose $src $dest
}

Related: differences documented in YSH Expressions vs. Python.

JavaScript

YSH uses JavaScript's dict literals:

var d1 = {name: 'Alice', age: 10}  # Keys aren't quoted

var d2 = {[mystr]: 'value'}        # Key expressions in []

var name = 'Bob'
var age = 15
var d3 = {name, age}  # Omitted values taken from surrounding scope

Blocks use curly braces, so most code resembles C / Java / JavaScript:

if (x > 0) {
  echo 'positive'
} else {
  echo 'zero or negative'
}

var i = 5
while (i > 0) {
  echo $i
  setvar i -= 1
}

Ruby

YSH has Ruby-like blocks:

cd /tmp {
  echo $PWD  # prints /tmp
}
echo $PWD

Perl

The @ character comes from Perl (and PowerShell):

var myarray = :| one two three |
echo @myarray          # @ is the "splice" operator

echo @[arrayfunc(x, y)]

for i in @(seq 3) {    # split command sub
  echo $i
}

Perl can be viewed as a mixture of shell, awk, and sed. YSH is a similar agglomeration of languages, but it's statically parsed.

Julia

The semicolon in proc and func definitions comes from Julia:

func f(x, y; invert=false) {
  if (invert) {
    return (-x - y)
  } else {
    return (x + y)
  }
}

Multiline strings in YSH strip leading whitespace, similar to Julia:

proc p {
  # Because leading and trailing space are stripped, this is 2 lines long
  var foods = '''
  peanut
  coconut
  '''
}

(Julia has something like blocks too.)

Go

Like Go, Oils is UTF-8-centric. (Go blog: Strings, bytes, runes and characters in Go.)

The design of for loops is roughly influenced by Go:

for i, item in (mylist) {  # ask for index and value
  echo "$i $item"
}

for i, k, v in (mydict) {  # ask for index, key, and value
  echo "$i $k $v"
}

Awk

YSH gets its regex match operator from Awk:

if (mystr ~ /digit+/) {
  echo 'Number'
}

(We don't use Perl's =~ operator.)

Lisp

YSH has "quotation types" that represent unevaluated code. Like Lisp, they give you control over evaluation:

var my_cmd = ^(ls /tmp | wc -l)
eval (my_cmd)

var my_expr = ^[42 + a[i]]
var v = evalExpr(my_expr)

var my_template = ^"hi $name"  # unimplemented

Haskell

YSH also uses ++ to concatenate strings and lists:

var mystr = a ++ b    
var mystr = "$a$b"       # very similar

var mylist = c ++ d
var mylist = :| @c @d |  # also converts every element to a string

YSH has a value.IO type that makes functions pure:

func renderPrompt(io) {
  return (io->promptVal('$') ++ " ")
}

Minor Influences

make, find and xargs

Our design for Ruby-like blocks was influenced by these mini-languages.

Tcl

YSH uses proc and setvar, which makes it look something like Tcl:

 proc p(x) {
   setvar y = x * 2
   echo $y
 }

 p 3  # prints 6

But this is mostly superficial: YSH isn't homoiconic like Tcl is, and has a detailed syntax. It intentionally avoids dynamic parsing.

However, Data Definition and Code Generation in Tcl (PDF) shows how Tcl can be used a configuration language:

change 6/11/2003 {
  author "Will Duquette"
  description {
    Added the SATl component to UCLO.
  }
}

Hay blocks in YSH allow this to be expressed very similarly:

hay define Change

Change 6/11/2003 {
  author = "Will Duquette"
  description = '''
    Added the SATl component to UCLO.
  '''
}

PHP

PHP has global variables like _REQUEST and _POST.

YSH has _status, _group(), _start(), etc. These are global variables that are "silently" mutated by the interpreter (and functions to access such global data).

Lua

YSH also uses a leading = to print expressions in the REPL.

= 1 + 2

Lua's implementation as a pure ANSI C core without I/O was also influential.

C

Most of our C-like syntax can be attributed to JavaScript or Python. But the value.Place type is created with the & operator, and should be familiar to C users:

$ echo hi | read --line (&myvar)
$ echo "myvar=$myvar"
=> myvar=hi

So a value.Place behaves like a pointer in some ways.

The & syntax may also feel familiar to Rust users.

Related

• Novelties in OSH and YSH