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authorTao Bao <tbao@google.com>2016-10-11 07:52:18 +0200
committerTao Bao <tbao@google.com>2016-10-13 08:29:59 +0200
commit39119ad8ecd00a9c19fb173c78cb4a8d22a4540a (patch)
treeb7c6d625baefc3521fe46c794d8d82fe99535229 /edify/README.md
parentMerge "updater: Kill the duplicate PrintSha1() in install.cpp." (diff)
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+edify
+=====
+
+Update scripts (from donut onwards) are written in a new little
+scripting language ("edify") that is superficially somewhat similar to
+the old one ("amend"). This is a brief overview of the new language.
+
+- The entire script is a single expression.
+
+- All expressions are string-valued.
+
+- String literals appear in double quotes. \n, \t, \", and \\ are
+ understood, as are hexadecimal escapes like \x4a.
+
+- String literals consisting of only letters, numbers, colons,
+ underscores, slashes, and periods don't need to be in double quotes.
+
+- The following words are reserved:
+
+ if then else endif
+
+ They have special meaning when unquoted. (In quotes, they are just
+ string literals.)
+
+- When used as a boolean, the empty string is "false" and all other
+ strings are "true".
+
+- All functions are actually macros (in the Lisp sense); the body of
+ the function can control which (if any) of the arguments are
+ evaluated. This means that functions can act as control
+ structures.
+
+- Operators (like "&&" and "||") are just syntactic sugar for builtin
+ functions, so they can act as control structures as well.
+
+- ";" is a binary operator; evaluating it just means to first evaluate
+ the left side, then the right. It can also appear after any
+ expression.
+
+- Comments start with "#" and run to the end of the line.
+
+
+
+Some examples:
+
+- There's no distinction between quoted and unquoted strings; the
+ quotes are only needed if you want characters like whitespace to
+ appear in the string. The following expressions all evaluate to the
+ same string.
+
+ "a b"
+ a + " " + b
+ "a" + " " + "b"
+ "a\x20b"
+ a + "\x20b"
+ concat(a, " ", "b")
+ "concat"(a, " ", "b")
+
+ As shown in the last example, function names are just strings,
+ too. They must be string *literals*, however. This is not legal:
+
+ ("con" + "cat")(a, " ", b) # syntax error!
+
+
+- The ifelse() builtin takes three arguments: it evaluates exactly
+ one of the second and third, depending on whether the first one is
+ true. There is also some syntactic sugar to make expressions that
+ look like if/else statements:
+
+ # these are all equivalent
+ ifelse(something(), "yes", "no")
+ if something() then yes else no endif
+ if something() then "yes" else "no" endif
+
+ The else part is optional.
+
+ if something() then "yes" endif # if something() is false,
+ # evaluates to false
+
+ ifelse(condition(), "", abort()) # abort() only called if
+ # condition() is false
+
+ The last example is equivalent to:
+
+ assert(condition())
+
+
+- The && and || operators can be used similarly; they evaluate their
+ second argument only if it's needed to determine the truth of the
+ expression. Their value is the value of the last-evaluated
+ argument:
+
+ file_exists("/data/system/bad") && delete("/data/system/bad")
+
+ file_exists("/data/system/missing") || create("/data/system/missing")
+
+ get_it() || "xxx" # returns value of get_it() if that value is
+ # true, otherwise returns "xxx"
+
+
+- The purpose of ";" is to simulate imperative statements, of course,
+ but the operator can be used anywhere. Its value is the value of
+ its right side:
+
+ concat(a;b;c, d, e;f) # evaluates to "cdf"
+
+ A more useful example might be something like:
+
+ ifelse(condition(),
+ (first_step(); second_step();), # second ; is optional
+ alternative_procedure())