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Oil's Expression Language: A Mix of Python and JavaScript

Recall that Oil is composed of three interleaved languages: words, commands, and expressions.

This doc describes expressions, but only the things that are not in:

• A Tour of the Oil Language. The best intro.
• The #expr-lang section of Oil Help Topics. A reference.
• Egg Expressions. A "sublanguage" this language.

TODO: This doc should have example shell sessions, like the tour does.

Preliminaries

Comparison to Python and JavaScript

For a short summary, see Oil vs. Python.

Constructs Shared Between Word and Expression Languages

All Substitutions: $myvar, $(hostname), etc.

Variable subs:

echo $myvar
var x = $myvar

Command subs:

echo $(hostname)
var x = $(hostname)  # no quotes necessary
var y = "name is $(hostname)"

String Literals:

echo 'foo'
var x = 'foo'
echo "hello $name"

Literals for Data Types

String Literals: Like Shell, But Less Confusion About Backslashes

Oil has 3 kinds of string literal. See the docs in the intro for detail, as well as the Strings doc.

As a detail, Oil disallows this case:

$ var x = '\n'
  var x = '\n'
           ^~
[ interactive ]:1: Strings with backslashes should look like r'\n' or $'\n'

In expression mode, you're forced to specify an explicit r or $ when the string has backslashes. This is because shell has the opposite default as Python: In shell, unadorned strings are raw. In Python, unadorned strings respect C escapes.

Float Literals

• Floating point literals are also like C/Python: 1.23e-10. Except:
  • A number is required before the . now
  • No 1_000_000.123_456 because that was hard to implement as a hand-written Python regex.

Those last two caveats about floats are TODOs: https://github.com/oilshell/oil/issues/483

List Type: Both "Array" and List Literals

There is a single list type, but it has two syntaxes:

• %(one two three) for an "array" of strings. This is equivalent to ['one', 'two', 'three'].
• [1, [2, 'three', {}]] for arbitrary Python-like "lists".

Longer example:

var x = %(a b c)
var x = %(
  'single quoted'
  "double quoted"
  $'c string'
  glob/*.py
  brace-{a,b,c}-{1..3}
)

Dict Literals Look Like JavaScript

Dict literals use JavaScript's rules, which are similar but not identical to Python.

The key can be either a bare word or bracketed expression.

(1) For example, {age: 30} means what {'age': 30} does in Python. That is, age is not the name of a variable. This fits more with the "dict as ad hoc struct" philosophy.

(2) In {[age]: 30}, age is a variable. You can put an arbitrary expression in there like {['age'.upper()]: 30}. (Note: Lua also has this bracketed key syntax.)

(3) {age, key2} is the same as {age: age, key2: key2}. That is, if the name is a bare word, you can leave off the value, and it will be looked up in the context where the dictionary is defined.

This is what ES2015 calls "shorthand object properties":

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer

Block, Expr, ArgList

TODO:

var myblock = ^(ls | wc -l)  

Operators on Multiple Types

Like JavaScript, Oil has two types of equality, but uses === and ~== rather than === and ==.

Exact Equality === !==

• TODO: types must be the same, so '42' === 42 is not just false, but it's an error.

Approximate Equality ~==

• There's no negative form like !==. Use not (a ~== b) instead.
• Valid Operand Types:
  • LHS: Str only
  • RHS: Str, Int, Bool

Examples:

' foo ' ~== 'foo'  # whitespace stripped on LEFT only
' 42 ' ~== 42
' TRue ' ~== true  # true, false, 0, 1, and I think T, F

Currently, there are no semantics for floats, so none of these work:

' 42.0 ' ~== 42
' 42 ' ~== 42.0
42.0 ~== 42
42 ~== 42.0

(Should float_equals() be a separate function?)

Function and Method Calls

var result = add(x, y)
var result = foo(x, named='default')

if (s.startswith('prefix')) {
  echo yes
}

Use Cases:

var d = {1: 2, 3: 4}
const k = keys(d)

Boolean Operators

Logical: not and or

Like Python.

Ternary

var cond = true
var x = 'yes' if cond else 'no'

Integer Operators

TODO: All of these do string -> int conversion, like

'1_000' => 1000   
'0xff' => 255
'0o010' => 8
'0b0001_0000' => 32

Arithmetic + - * / // % and **

Like Python.

Bitwise ~ & | ^ << >>

Like Python.

Comparison on Integers and Floats < <= > >=

• NOTE:
  • do we have is and is not? Not sure I want identity in the language?
  • is everything nullable too?

https://github.com/oilshell/oil/blob/master/spec/oil-expr.test.sh#L550

if (1 < 2 <= 2 <= 3 < 4) {
  echo '123'
}

This syntax is directly from Python. That is,

x op y op z

is a shortcut for

x op y and y op z

Comments welcome!

String Pattern Matching ~ and ~~

• Eggex: ~ !~
  • Similar to bash's [[ $x =~ $pat ]]
• Glob: ~~ !~~
  • Similar to bash's [[ $x == *.py ]]

String and List Operators

In addition to pattern matching.

Concatenation with ++

s ++ 'suffix'
L ++ [1, 2] ++ %(a b)

Indexing a[i]

var s = 'foo'
var second = s[1]    # are these integers though?  maybe slicing gives you things of length 1
echo $second  # 'o'

var a = %(spam eggs ham)
var second = a[1]
echo $second  # => 'eggs'

echo $[a[-1]]  # => ham

Semantics are like Python: Out of bounds is an error.

Slicing a[i:j]

var s = 'food'
var slice = s[1:3]
echo $second  # 'oo'

var a = %(spam eggs ham)
var slice = a[1:3]
write -- @slice  # eggs, ham

Semantics are like Python: Out of bounds is not an error.

Dict Operators

Membership with in

• And not in
• But strings and arrays use functions?
  • .find() ? It's more of an algorithm.

d->key is a shortcut for d['key']

the distinction between attributes and dictionary members always seemed weird and unnecessary to me.

I've been thinking about this for the Oil language, which is heavily influenced by Python.

The problem is that dictionary attributes come from user data, i.e. from JSON, while methods like .keys() come from the interpreter, and Python allows you to provide user-defined methods like mydict.mymethod() too.

Mixing all of those things in the same namespace seems like a bad idea.

In Oil I might do introduce an -> operator, so d->mykey is a shortcut for d['mykey'].

d.keys(), d.values(), d.items()  # methods
d->mykey
d['mykey']

Maybe you could disallow user-defined attributes on dictionaries, and make them free:

keys(d), values(d), items(d)
d.mykey  # The whole namespace is available for users

However I don't like that this makes dictionaries a special case. Thoughts?

Deferred

List and Dict Comprehensions

List comprehensions might be useful for a "faster" for loop? It only does expressions?

Splat * and **

Python allows splatting into lists:

a = [1, 2] 
b = [*a, 3]

And dicts:

d = {'name': 'alice'}
d2 = {**d, age: 42}

Ranges 1:n (vs slices)

Deferred because you can use

for i in @(seq $n) {
  echo $i
}

This gives you strings but that's OK for now. We don't yet have a "fast" for loop.

Notes:

• Oil slices don't have a "step" argument. Justification:
  • R only has start:end, it doesn't have start:end:step
  • Julia has start:step:end!
  • I don't think the step is so useful that it has to be first class syntax. In other words, Python's syntax is optimized for a rare case -- e.g. a[::2].
• Python has slices, but it doesn't have a range syntax. You have to write range(0, n).
• A syntactic difference between slices and ranges: slice endpoints can be implicit, like a[:n] and a[3:].

Appendices

Oil vs. Tea

• Tea: truthiness of Str* is a problem. Nul, etc.
  • if (mystr) vs if (len(mystr))
  • though I think strings should be non-nullable value types? They are slices.
  • they start off as the empty slice
• Coercions of strings to numbers
  • 42 and 3.14 and 1e100

Implementation Notes

• Limitation:
  • Start with Str, StrArray, and AssocArray data model
  • Then add int, float, bool, null (for JSON)
  • Then add fully recursive data model (depends on FC)
    • value = ... | dict[str, value]