• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1Update scripts (from donut onwards) are written in a new little
2scripting language ("edify") that is superficially somewhat similar to
3the old one ("amend").  This is a brief overview of the new language.
4
5- The entire script is a single expression.
6
7- All expressions are string-valued.
8
9- String literals appear in double quotes.  \n, \t, \", and \\ are
10  understood, as are hexadecimal escapes like \x4a.
11
12- String literals consisting of only letters, numbers, colons,
13  underscores, slashes, and periods don't need to be in double quotes.
14
15- The following words are reserved:
16
17       if    then    else   endif
18
19  They have special meaning when unquoted.  (In quotes, they are just
20  string literals.)
21
22- When used as a boolean, the empty string is "false" and all other
23  strings are "true".
24
25- All functions are actually macros (in the Lisp sense); the body of
26  the function can control which (if any) of the arguments are
27  evaluated.  This means that functions can act as control
28  structures.
29
30- Operators (like "&&" and "||") are just syntactic sugar for builtin
31  functions, so they can act as control structures as well.
32
33- ";" is a binary operator; evaluating it just means to first evaluate
34  the left side, then the right.  It can also appear after any
35  expression.
36
37- Comments start with "#" and run to the end of the line.
38
39
40
41Some examples:
42
43- There's no distinction between quoted and unquoted strings; the
44  quotes are only needed if you want characters like whitespace to
45  appear in the string.  The following expressions all evaluate to the
46  same string.
47
48     "a b"
49     a + " " + b
50     "a" + " " + "b"
51     "a\x20b"
52     a + "\x20b"
53     concat(a, " ", "b")
54     "concat"(a, " ", "b")
55
56  As shown in the last example, function names are just strings,
57  too.  They must be string *literals*, however.  This is not legal:
58
59     ("con" + "cat")(a, " ", b)         # syntax error!
60
61
62- The ifelse() builtin takes three arguments:  it evaluates exactly
63  one of the second and third, depending on whether the first one is
64  true.  There is also some syntactic sugar to make expressions that
65  look like if/else statements:
66
67     # these are all equivalent
68     ifelse(something(), "yes", "no")
69     if something() then yes else no endif
70     if something() then "yes" else "no" endif
71
72  The else part is optional.
73
74     if something() then "yes" endif    # if something() is false,
75                                        # evaluates to false
76
77     ifelse(condition(), "", abort())   # abort() only called if
78                                        # condition() is false
79
80  The last example is equivalent to:
81
82     assert(condition())
83
84
85- The && and || operators can be used similarly; they evaluate their
86  second argument only if it's needed to determine the truth of the
87  expression.  Their value is the value of the last-evaluated
88  argument:
89
90     file_exists("/data/system/bad") && delete("/data/system/bad")
91
92     file_exists("/data/system/missing") || create("/data/system/missing")
93
94     get_it() || "xxx"     # returns value of get_it() if that value is
95                           # true, otherwise returns "xxx"
96
97
98- The purpose of ";" is to simulate imperative statements, of course,
99  but the operator can be used anywhere.  Its value is the value of
100  its right side:
101
102     concat(a;b;c, d, e;f)     # evaluates to "cdf"
103
104  A more useful example might be something like:
105
106     ifelse(condition(),
107            (first_step(); second_step();),   # second ; is optional
108            alternative_procedure())
109