Introduce function definition, and add more examples.

jq.html.markdown

@@ -362,7 +362,7 @@ jq -n '.unknown_key // 7' # => 7
 jq -n '123 | .[0]'     # => jq: error (at <unknown>): Cannot index number with number
 jq -n '"abc" | .name'  # => jq: error (at <unknown>): Cannot index string with string "name"
 jq -n '{"a": 97} | .[0]'    # => jq: error (at <unknown>): Cannot index object with number
-jq -n '[89, 64] | .["key"]' # => jq: error (at <unknown>): Cannot index array with string "key" 
+jq -n '[89, 64] | .["key"]' # => jq: error (at <unknown>): Cannot index array with string "key"
 
 # You can, however, append a `?` to a lookup to make jq return `empty`
 # instead when such error happens.
@@ -443,7 +443,19 @@ jq -n '{ values: ({ a: 1, b: 2, c: 3 }[] | . * 2) }'
 
 
 # Conditional `if ... then ... else ... end` in jq is an expression, so
-# both the `then` part and the `else` part are required.
+# both the `then` part and the `else` part are required. In jq, only
+# two values, `null` and `false`, are false; all other values are true.
+#
+jq -n 'if 1 > 2 | not and 1 <= 2 then "Makes sense" else "WAT?!" end'
+
+# Output
+# "Makes sense"
+
+# Notice that `not` is a built-in function that takes zero arguments,
+# that's why it's used as a filter to negate its input value.
+# We'll talk about functions soon.
+
+# Another example using a conditional:
 #
 jq -n '1, 2, 3, 4, 5 | if . % 2 != 0 then . else empty end'
 
@@ -608,7 +620,7 @@ echo $numbers | jq -rs '  # Slurp the numbers into an array.
   |                        # For each object, generate two lines:
   "Group \(.key): \(.value | sort | join(" "))"   + "\n" +
   "Average: \(      .value | (add / length)  )"
-    
+
 ] # Contain the group+average lines in an array.
   # Join the array elements by separator lines (dashes) to produce the report.
 | join("\n" + "-"*78 + "\n")
@@ -682,7 +694,7 @@ jq -n '["a", "b", "c"] | reduce .[] as $i (""; . + $i)'  # => "abc"
 #
 #    reduce (1, 2, 3, 4, 5) as $i (0; . + $i)
 #
-# can be think of as doing: 
+# can be think of as doing:
 #
 #    0 + 1 | . + 2 | . + 3 | . + 4 | . + 5
 #
@@ -730,7 +742,7 @@ jq -rn '[1, 2, 3, 4, 5]
 # With the `expr as $var` form, if multiple values are generated by `expr`
 # then jq will iterate through them and bind each value to `$var` in turn
 # for the rest of the pipeline.
-# 
+#
 jq -rn 'range(2; 4) as $i
         | range(1; 6) as $j
           | "\($i) * \($j) = \($i * $j)"
@@ -749,6 +761,26 @@ jq -rn 'range(2; 4) as $i
 # 3 * 5 = 15
 
 
+# It's sometimes useful to bind the initial input to a variable at the
+# start of a program, so that you can refer to it later down the pipeline.
+#
+jq -rn "$(cat <<'EOF'
+    {lookup:  {a: 1, b: 2, c: 3},
+     bonuses: {a: 5, b: 2, c: 9}
+    }
+    | . as $doc
+    | .bonuses
+    | to_entries[]
+    | "\(.key)'s total is \($doc.lookup[.key] + .value)"
+EOF
+)"
+
+# Output:
+# a's total is 6
+# b's total is 4
+# c's total is 12
+
+
 # In jq, values can be assigned to an array index or object key via the
 # assignment operator, `=`. The same current input is given to both sides
 # of the assignment operator, and the assignment itself evaluates to the
@@ -761,7 +793,7 @@ jq -n '.a = 1 | .b = .a + 1'  # => {"a": 1, "b": 2}
 # filter, and assiging to a key under `null` turns it into an object with
 # the key. The same input (now an object) then gets piped to the next filter,
 # which then sets the `b` key to the value of the `a` key plus `1`, which is `2`.
-# 
+#
 
 # Another example:
 #
@@ -799,9 +831,43 @@ jq -n '{a: 1, b: {c: 2}, d: [3, 4, 5]} | del(.b.c, .d[1]) | .b.x = 6'
 #   ]
 # }
 
-# FIXME: Cover more topics
-# - function definition
-# - ...
+
+# Other than using jq's built-in functions, you can define your own.
+# In fact, many built-in functions are defined using jq (see the link
+# to jq's built-in functions at the end of the doc).
+#
+jq -n '
+    def my_select(expr): if expr then . else empty end;
+    def my_map(expr): [.[] | expr];
+    def sum: reduce .[] as $x (0; . + $x);
+    def my_range($from; $to):
+        if $from >= $to then
+            empty
+        else
+            $from, my_range($from + 1; $to)
+        end
+    ;
+    [my_range(1; 6)] | my_map(my_select(. % 2 != 0)) | sum
+'
+
+# Output:
+# 9
+
+# Some notes about function definitons:
+#
+# - Functions are usually defined at the beginning, so that they are available
+#   to the rest of the jq program.
+#
+# - Each function definion should end with a `;` (semicolon).
+#
+# - It's also possible to define a function within another, though it's not shown here.
+#
+# - Function parameters are separated by `;` (semicolor). This is consistent with
+#   passing multiple arguments when calling a function.
+#
+# - `def f($a; $b): ...;` is a shorthand for: `def f(a; b): a as $a | b as $b | ...`
+#
+
 ```
 
 ## Further Reading