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YSH Expressions vs. Python

The YSH expression language borrows heavily from Python. In fact, it literally started with Python's Grammar/Grammar file.

This doc describes the differences, which may help Python users learn YSH.

If you don't know Python, A Tour of YSH explains the language from the clean-slate perpsective.

(TODO: A separate doc could compare commands/statements like for and if.)

Background

YSH has dynamic types, much like Python. These are the main data types:

Null Bool Int Float List Dict

Quick example:

var x = null
var y = f(true, 42, 3.14)
var z = [5, 6, {name: 'bob'}]

Literals

Every data type can be written as a literal. Literals generally look like Python, so this section describes what's the same, and what's changed / added / and removed.

Like Python: numbers and lists

• Integers: 123, 1_000_000, 0b1100_0010, 0o755, 0xff
• Floats: 1.023e6
• Lists: ['pea', 'nut']
  • TODO: we want Python-like list comprehensions

Changed: booleans, strings, dicts

• Atoms are true, false, and null (like JavaScript) rather than True, False, and None (like Python).

  • In YSH, we use capital letters for types like Int.

• String literals are like shell string literals, not like Python.

  • Double Quoted: "hello $name"
  • Single quoted: r'c:\Program Files\'
  • C-style: $'line\n' (TODO: change to J8 Notation)
    • Unicode literals are \u{3bc} instead of \u03bc and \U000003bc

• Dicts use JavaScript syntax, not Python syntax.

  • Unquoted keys: {age: 42}
  • Bracketed keys: {[myvar + 1]: 'value'}
  • "Punning": {age}

Added

• Shell-like list literals: :| pea nut | is equivalent to ['pea', 'nut']

• "Quotation" types for unevaluated code:

  • Command / block ^(ls | wc -l)
  • Unevaluated expression ^[1 + a[i] + f(x)]

• Units on number constants like 100 MiB (reserved, not implemented)

Omitted

• YSH has no tuples, only lists.
• No lambdas (function literals returning an expression)
• No closures, or scope declarations like global and nonlocal. (We would prefer objects over closures.)
• No iterators.
  • Instead we have for loop that works on lists and dicts.
  • It flexibly accepts up to 3 loop variables, taking the place of Python's enumerate(), keys(), values(), and items().

Operators

Like literals, YSH operators resemblle Python operators. The expression 42 + a[i] means the same thing in both languages.

This section describes what's the same, and what's changed / added / removed.

Note: YSH Does Less Operator Overloading

YSH doesn't overload operators as much because it often does automatic Str ↔ Int conversions (like Awk):

• a + b is for addition, while a ++ b is for concatenation.

• a < b does numeric comparison, not lexicographical comparison of strings.

  • (We should add strcmp() for strings.)

Like Python

• Arithmetic + - * / and comparison < > <= =>. They also convert strings to integers or floats. Examples:

  • '22' < '3' is true because 22 < 3 is true.
  • '3.1' <= '3.14' is true because 3.1 <= 3.14 is true.

• Integer arithmetic: // integer division, % modulus, ** exponentiation.

  • They also convert strings to integers (but not floats).

• Bitwise & | ~ ^ << >>

• Logical and or not

• Ternary 0 if cond else 1

• Slicing: s[i:j] evaluates to a string

• Membership in, not in

• Identity is, is not

• Function Call: f(x, y)

Changed

• Equality is === !==, because we also have ~==.
• String Concatenation is ++, not +. Again, + is always addition.
• Splat operator is ... not *: f(...myargs).

Added

• Eggex match s ~ /d+/
• Glob match s ~~ '*.py'
• Approximate Equality 42 ~== '42'
• YSH sigils: $ and @
• mydict.key as an alias for mydict['key']

Omitted

• No string formatting with %. Use ${x %.3f} instead. (unimplemented)
• No @ for matrix multiply.
• I removed slice step syntax 1:5:2 because 0::2 could conflict with module::name (could be restored).

Syntax Compared With JavaScript

This section may be useful if yo know JavaScript.

• YSH uses === and ~== for exact and type-converting equality, while JS uses === and ==.

• Expressions are more like Python:

  • YSH expressions use and or not while JS uses && || !. In shell, && || ! are already used in the command language (but they're somewhat less important than in YSH).
  • The YSH ternary operator is 0 if cond else 1, while in JS it's cond ? 0 : 1.
  • Operator precedence rules are slightly different, but still C-like. They follow Python's grammar.

• Same differences as above, versus Python:

  • s ++ t for string concatenation rather than s + t
  • Shell string literals rather than JS string literals

Semantics Compared

The runtime behavior of YSH is also similar to Python and JavaScript.

Versus Python

• Bool and Int are totally separate types. YSH is like JavaScript, where they aren't equal: true !== 1. In Python, they are equal: True == 1.

• Strings are bytes, which may UTF-8 encoded, like Go. (In Python 3, strings are sequences of code points, which are roughly integers up to 2²¹.)

• We avoid operators that cause "accidentally quadratic" behavior.

  • No in on List, since that's a linear search. Only in on Dict.
  • The're not ++= operator on strings.

Versus JavaScript

• Strings are bytes, which may UTF-8 encoded, like Go. (In JavaScript are sequences of UTF-16 code units, which are roughly integers up to 2¹⁶.)
• Undefined variables result in a fatal error like Python, not a silently propagating undefined like JavaScript.