1 | ---
|
2 | in_progress: yes
|
3 | css_files: ../../web/base.css ../../web/manual.css ../../web/toc.css
|
4 | ---
|
5 |
|
6 | Oil's Expression Language: A Mix of Python and JavaScript
|
7 | =========================================================
|
8 |
|
9 | Recall that Oil is composed of three interleaved languages:
|
10 | [words](word-language.html), [commands](command-language.html), and
|
11 | **expressions**.
|
12 |
|
13 | This doc describes expressions, but only the things that are **not** in:
|
14 |
|
15 | - [A Tour of the Oil Language](oil-language-tour.html). The best intro.
|
16 | - The `#expr-lang` section of [Oil Help
|
17 | Topics](oil-help-topics.html#expr-lang). A reference.
|
18 | - [Egg Expressions](eggex.html). A "sublanguage" this language.
|
19 |
|
20 | TODO: This doc should have example shell sessions, like the tour does.
|
21 |
|
22 | <div id="toc">
|
23 | </div>
|
24 |
|
25 | ## Preliminaries
|
26 |
|
27 | ### Comparison to Python and JavaScript
|
28 |
|
29 | For a short summary, see [Oil vs. Python](oil-vs-python.html).
|
30 |
|
31 | ### Constructs Shared Between Word and Expression Languages
|
32 |
|
33 | String literals can be used in both words and expressions:
|
34 |
|
35 | echo 'foo'
|
36 | var x = 'foo'
|
37 |
|
38 | echo "hello $name"
|
39 | var y = "hello $name"
|
40 |
|
41 | echo $'\t TAB'
|
42 | var z = $'\t TAB'
|
43 |
|
44 | This includes multi-line string literals:
|
45 |
|
46 | echo '''
|
47 | hello
|
48 | world
|
49 | '''
|
50 |
|
51 | var x = '''
|
52 | hello
|
53 | world
|
54 | '''
|
55 |
|
56 | # (and the 2 other kinds)
|
57 |
|
58 | Command substitution is shared:
|
59 |
|
60 | echo $(hostname)
|
61 | var a = $(hostname) # no quotes necessary
|
62 | var b = "name is $(hostname)"
|
63 |
|
64 | String substitution is shared:
|
65 |
|
66 | echo ${MYVAR:-}
|
67 | var c = ${MYVAR:-}
|
68 | var d = "var is ${MYVAR:-}"
|
69 |
|
70 | Not shared:
|
71 |
|
72 | - Unquoted substitution `$foo` isn't available in expression mode. (It should
|
73 | be or bare `foo`, or `"$foo"`)
|
74 | - Expression sub `$[1 + 2]` is usually not necessary in expression mode, so it
|
75 | isn't available. You can use a quoted string like `var x = "$[1 + 2]"`.
|
76 |
|
77 | ## Literals for Data Types
|
78 |
|
79 | ### String Literals: Like Shell, But Less Confusion About Backslashes
|
80 |
|
81 | Oil has 3 kinds of string literal. See the docs in the intro for detail, as
|
82 | well as the [Strings](strings.html) doc.
|
83 |
|
84 | As a detail, Oil disallows this case:
|
85 |
|
86 | $ var x = '\n'
|
87 | var x = '\n'
|
88 | ^~
|
89 | [ interactive ]:1: Strings with backslashes should look like r'\n' or $'\n'
|
90 |
|
91 | In expression mode, you're forced to specify an explicit `r` or `$` when the
|
92 | string has backslashes. This is because shell has the opposite default as
|
93 | Python: In shell, unadorned strings are raw. In Python, unadorned strings
|
94 | respect C escapes.
|
95 |
|
96 | ### Float Literals
|
97 |
|
98 | - Floating point literals are also like C/Python: `1.23e-10`. Except:
|
99 | - A number is required before the `.` now
|
100 | - No `1_000_000.123_456` because that was hard to implement as a hand-written
|
101 | Python regex.
|
102 |
|
103 | Those last two caveats about floats are TODOs:
|
104 | <https://github.com/oilshell/oil/issues/483>
|
105 |
|
106 | ### List Type: Both "Array" and List Literals
|
107 |
|
108 | There is a single list type, but it has two syntaxes:
|
109 |
|
110 | - `:| one two three |` for an "array" of strings. This is equivalent to
|
111 | `['one', 'two', 'three']`.
|
112 | - `[1, [2, 'three', {}]]` for arbitrary Python-like "lists".
|
113 |
|
114 | Longer example:
|
115 |
|
116 | var x = :| a b c |
|
117 | var x = :|
|
118 | 'single quoted'
|
119 | "double quoted $var"
|
120 | $'c string'
|
121 | glob/*.py
|
122 | brace-{a,b,c}-{1..3}
|
123 | |
|
124 |
|
125 | ### Dict Literals Look Like JavaScript
|
126 |
|
127 | Dict literals use JavaScript's rules, which are similar but not identical to
|
128 | Python.
|
129 |
|
130 | The key can be either a **bare word** or **bracketed expression**.
|
131 |
|
132 | (1) For example, `{age: 30}` means what `{'age': 30}` does in Python. That is,
|
133 | `age` is **not** the name of a variable. This fits more with the "dict as ad
|
134 | hoc struct" philosophy.
|
135 |
|
136 | (2) In `{[age]: 30}`, `age` is a variable. You can put an arbitrary expression
|
137 | in there like `{['age'.upper()]: 30}`. (Note: Lua also has this bracketed key
|
138 | syntax.)
|
139 |
|
140 | (3) `{age, key2}` is the same as `{age: age, key2: key2}`. That is, if the
|
141 | name is a bare word, you can leave off the value, and it will be looked up in
|
142 | the context where the dictionary is defined.
|
143 |
|
144 | This is what ES2015 calls "shorthand object properties":
|
145 |
|
146 | - <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer>
|
147 |
|
148 | ### Block, Expr
|
149 |
|
150 | TODO:
|
151 |
|
152 | var myblock = ^(ls | wc -l)
|
153 | var myexpr = ^[1 + 2]
|
154 |
|
155 | ## Operators on Multiple Types
|
156 |
|
157 | Like JavaScript, Oil has two types of equality, but uses `===` and `~==` rather
|
158 | than `===` and `==`.
|
159 |
|
160 | ### Exact Equality `=== !==`
|
161 |
|
162 | - TODO: types must be the same, so `'42' === 42` is not just false, but it's an
|
163 | **error**.
|
164 |
|
165 | ### Approximate Equality `~==`
|
166 |
|
167 | - There's no negative form like `!==`. Use `not (a ~== b)` instead.
|
168 | - Valid Operand Types:
|
169 | - LHS: `Str` only
|
170 | - RHS: `Str`, `Int`, `Bool`
|
171 |
|
172 | Examples:
|
173 |
|
174 | ' foo ' ~== 'foo' # whitespace stripped on LEFT only
|
175 | ' 42 ' ~== 42
|
176 | ' TRue ' ~== true # true, false, 0, 1, and I think T, F
|
177 |
|
178 | Currently, there are no semantics for floats, so none of these work:
|
179 |
|
180 | ' 42.0 ' ~== 42
|
181 | ' 42 ' ~== 42.0
|
182 | 42.0 ~== 42
|
183 | 42 ~== 42.0
|
184 |
|
185 | (Should `float_equals()` be a separate function?)
|
186 |
|
187 | ### Function and Method Calls
|
188 |
|
189 | var result = add(x, y)
|
190 | var result = foo(x, named='default')
|
191 |
|
192 | if (s.startswith('prefix')) {
|
193 | echo yes
|
194 | }
|
195 |
|
196 | Use Cases:
|
197 |
|
198 | var d = {1: 2, 3: 4}
|
199 | const k = keys(d)
|
200 |
|
201 |
|
202 | ## Boolean Operators
|
203 |
|
204 | ### Logical: `not` `and` `or`
|
205 |
|
206 | Like Python.
|
207 |
|
208 | ### Ternary
|
209 |
|
210 | var cond = true
|
211 | var x = 'yes' if cond else 'no'
|
212 |
|
213 | ## Arithmetic
|
214 |
|
215 | <!--
|
216 | TODO: Should the string to number/integer conversions also handle these cases?
|
217 |
|
218 | '1_000' => 1000
|
219 | '0xff' => 255
|
220 | '0o010' => 8
|
221 | '0b0001_0000' => 32
|
222 |
|
223 | Right now comparison operators convert decimal strings.
|
224 | -->
|
225 |
|
226 | ### Arithmetic `+ - * /`
|
227 |
|
228 | These are like Python, but they do string to number conversion (but not unary
|
229 | `-`.) A number is an integer or float.
|
230 |
|
231 | That is:
|
232 |
|
233 | - `'1' + '2'` evaluates to `3` because `1 + 2` evaluates to `3`.
|
234 | - `'1' + '2.5'` evaluates to `3.5` because `1 + 2.5` evaluates to `3.5`.
|
235 |
|
236 | ### Arithmetic `// %` and `**`
|
237 |
|
238 | Also like Python, but they do string to **integer** conversion.
|
239 |
|
240 | - `'9' // '4'` evaluates to `2` because `9 / 4` evaluates to `2`.
|
241 |
|
242 | ### Bitwise `~ & | ^ << >>`
|
243 |
|
244 | Like Python.
|
245 |
|
246 | ## Comparison of Integers and Floats `< <= > >=`
|
247 |
|
248 | These operators also do string to number conversion. That is:
|
249 |
|
250 | - `'22' < '3'` false because `22 < 3` is false. (It would be true under
|
251 | lexicographical comparison.)
|
252 | - `'3.1' <= '3.14'` is true because `3.1 <= 3.14` is true.
|
253 |
|
254 | TODO:
|
255 |
|
256 | - Do we have `is` and `is not`? I think it's useful for lists and dicts
|
257 | - Remove chained comparison? This syntax is directly from Python.
|
258 | - That is, `x op y op z` is a shortcut for `x op y and y op z`
|
259 |
|
260 | ## String Pattern Matching `~` and `~~`
|
261 |
|
262 | - Eggex: `~` `!~`
|
263 | - Similar to bash's `[[ $x =~ $pat ]]`
|
264 | - Glob: `~~` `!~~`
|
265 | - Similar to bash's `[[ $x == *.py ]]`
|
266 |
|
267 | ## String and List Operators
|
268 |
|
269 | In addition to pattern matching.
|
270 |
|
271 | ### Concatenation with `++`
|
272 |
|
273 | s ++ 'suffix'
|
274 | L ++ [1, 2] ++ :| a b |
|
275 |
|
276 | ### Indexing `a[i]`
|
277 |
|
278 | var s = 'foo'
|
279 | var second = s[1] # are these integers though? maybe slicing gives you things of length 1
|
280 | echo $second # 'o'
|
281 |
|
282 | var a = :| spam eggs ham |
|
283 | var second = a[1]
|
284 | echo $second # => 'eggs'
|
285 |
|
286 | echo $[a[-1]] # => ham
|
287 |
|
288 | Semantics are like Python: Out of bounds is an error.
|
289 |
|
290 | ### Slicing `a[i:j]`
|
291 |
|
292 | var s = 'food'
|
293 | var slice = s[1:3]
|
294 | echo $second # 'oo'
|
295 |
|
296 | var a = :| spam eggs ham |
|
297 | var slice = a[1:3]
|
298 | write -- @slice # eggs, ham
|
299 |
|
300 | Semantics are like Python: Out of bounds is **not** an error.
|
301 |
|
302 | ## Dict Operators
|
303 |
|
304 | ### Membership with `in`
|
305 |
|
306 | - And `not in`
|
307 | - But strings and arrays use functions?
|
308 | - .find() ? It's more of an algorithm.
|
309 |
|
310 | ### `d->key` is a shortcut for `d['key']`
|
311 |
|
312 | > the distinction between attributes and dictionary members always seemed weird
|
313 | > and unnecessary to me.
|
314 |
|
315 | I've been thinking about this for [the Oil
|
316 | language](http://www.oilshell.org/blog/2019/08/22.html), which is heavily
|
317 | influenced by Python.
|
318 |
|
319 | The problem is that dictionary attributes come from user data, i.e. from JSON,
|
320 | while methods like `.keys()` come from the interpreter, and Python allows you
|
321 | to provide user-defined methods like `mydict.mymethod()` too.
|
322 |
|
323 | Mixing all of those things in the same namespace seems like a bad idea.
|
324 |
|
325 | In Oil I might do introduce an `->` operator, so `d->mykey` is a shortcut for
|
326 | `d['mykey']`.
|
327 |
|
328 | ```
|
329 | d.keys(), d.values(), d.items() # methods
|
330 | d->mykey
|
331 | d['mykey']
|
332 | ```
|
333 |
|
334 | Maybe you could disallow user-defined attributes on dictionaries, and make them
|
335 | free:
|
336 |
|
337 | ```
|
338 | keys(d), values(d), items(d)
|
339 | d.mykey # The whole namespace is available for users
|
340 | ```
|
341 |
|
342 | However I don't like that this makes dictionaries a special case. Thoughts?
|
343 |
|
344 | ## Deferred
|
345 |
|
346 | ### List and Dict Comprehensions
|
347 |
|
348 | List comprehensions might be useful for a "faster" for loop? It only does
|
349 | expressions?
|
350 |
|
351 | ### Splat `*` and `**`
|
352 |
|
353 | Python allows splatting into lists:
|
354 |
|
355 | a = [1, 2]
|
356 | b = [*a, 3]
|
357 |
|
358 | And dicts:
|
359 |
|
360 | d = {'name': 'alice'}
|
361 | d2 = {**d, age: 42}
|
362 |
|
363 | ### Ranges `1:n` (vs slices)
|
364 |
|
365 | Deferred because you can use
|
366 |
|
367 | for i in @(seq $n) {
|
368 | echo $i
|
369 | }
|
370 |
|
371 | This gives you strings but that's OK for now. We don't yet have a "fast" for
|
372 | loop.
|
373 |
|
374 | Notes:
|
375 |
|
376 | - Oil slices don't have a "step" argument. Justification:
|
377 | - R only has `start:end`, it doesn't have `start:end:step`
|
378 | - Julia has `start:step:end`!
|
379 | - I don't think the **step** is so useful that it has to be first class
|
380 | syntax. In other words, Python's syntax is optimized for a rare case --
|
381 | e.g. `a[::2]`.
|
382 | - Python has slices, but it doesn't have a range syntax. You have to write
|
383 | `range(0, n)`.
|
384 | - A syntactic difference between slices and ranges: slice endpoints can be
|
385 | **implicit**, like `a[:n]` and `a[3:]`.
|
386 |
|
387 | ## Appendices
|
388 |
|
389 | ### Oil vs. Tea
|
390 |
|
391 | - Tea: truthiness of `Str*` is a problem. Nul, etc.
|
392 | - `if (mystr)` vs `if (len(mystr))`
|
393 | - though I think strings should be non-nullable value types? They are
|
394 | slices.
|
395 | - they start off as the empty slice
|
396 | - Automatic conversions of strings to numbers
|
397 | - `42` and `3.14` and `1e100`
|
398 |
|
399 | ### Implementation Notes
|
400 |
|
401 | - Limitation:
|
402 | - Start with Str, StrArray, and AssocArray data model
|
403 | - Then add int, float, bool, null (for JSON)
|
404 | - Then add fully recursive data model (depends on FC)
|
405 | - `value = ... | dict[str, value]`
|
406 |
|