clarify type casting in CESQL spec (#1281)

* clarify type casting in CESQL spec

Signed-off-by: Calum Murray <[email protected]>

* fix: addressed feedback from meeting

Signed-off-by: Calum Murray <[email protected]>

* address review feedback

Signed-off-by: Calum Murray <[email protected]>

* clarify the default values when an error occurs

Signed-off-by: Calum Murray <[email protected]>

* fix: may -> MAY

Signed-off-by: Calum Murray <[email protected]>

* clarify second operand in LIKE expression MUST be a string literal

Signed-off-by: Calum Murray <[email protected]>

* clarify leading zeros

Signed-off-by: Calum Murray <[email protected]>

* add table of supported and unsupported type casts

Signed-off-by: Calum Murray <[email protected]>

* add boolean <-> integer casting

Signed-off-by: Calum Murray <[email protected]>

---------

Signed-off-by: Calum Murray <[email protected]>

cesql/spec.md

@@ -268,7 +268,7 @@ Similarly, whenever the left operand of the OR operation evaluates to `true`, th
 | `x LIKE pattern: String x String -> Boolean`     | Returns `true` if the value x matches the `pattern` |
 | `x NOT LIKE pattern: String x String -> Boolean` | Same as `NOT (x LIKE PATTERN)`                      |
 
-The pattern of the `LIKE` operator can contain:
+The pattern of the `LIKE` operator MUST be a string literal, and can contain:
 
 - `%` represents zero, one, or multiple characters
 - `_` represents a single character
@@ -278,6 +278,10 @@ For example, the pattern `_b*` will accept values `ab`, `abc`, `abcd1` but won't
 Both `%` and `_` can be escaped with `\`, in order to be matched literally. For example, the pattern `abc\%` will match
 `abc%` but won't match `abcd`.
 
+In cases where the left operand is not a `String`, it MUST be cast to a `String` before the comparison is made. 
+The pattern of the `LIKE` operator (that is, the right operand of the operator) MUST be a valid string literal without casting,
+otherwise the parser MUST return a parse error.
+
 #### 3.4.4. Exists operator
 
 | Definition                          | Semantics                                                                   |
@@ -353,6 +357,35 @@ left operand of the OR operation evalues to `true`, the right operand MUST NOT b
 
 #### 3.7. Type casting
 
+The following table indicates which type casts a CESQL engine MUST or MUST NOT support:
+
+| Type    | Integer | String | Boolean |
+| ------- | ------- | ------ | ------- |
+| Integer | N/A     | MUST   | MUST    |
+| String  | MUST    | N/A    | MUST    |
+| Boolean | MUST    | MUST   | N/A     |
+
+For all of the type casts which a CESQL engine MUST support, the semantics which the engine MUST use are defined as follows:
+
+| Definition           | Semantics                                                                                                                                                                                                                                         |
+| --------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
+| `Integer -> String`   | Returns the string representation of the integer value in base 10, without leading `0`s. If the value is less than 0, the '-' character is prepended to the result.                                                                              |
+| `Integer -> Boolean`  | Returns `false` if the integer is `0`, and `true` otherwise.                                                                              |
+| `String -> Integer`   | Returns the result of interpreting the string as a 32 bit base 10 integer. The string MAY begin with a leading sign '+' or '-'. If the result will overflow or the string is not a valid integer an error is returned along with a value of `0`. |
+| `String -> Boolean`   | Returns `true` or `false` if the lower case representation of the string is exactly "true" or "false, respectively. Otherwise returns an error along with a value of `false`                                                                     |
+| `Boolean -> Integer`  | Returns `1` if the boolean is `true`, and `0` if the boolean is `false`.                                                                                                                                                                         |
+| `Boolean -> String`   | Returns `"true"` if the boolean is `true`, and `"false"` if the boolean is `false`.                                                                                                                                                              |
+
+An example of how _Boolean_ values cast to _String_ combines with the case insensitivity of CESQL keywords is that:
+```
+TRUE = "true" AND FALSE = "false"
+```
+will evaluate to `true`, while
+```
+TRUE = "TRUE" OR FALSE = "FALSE"
+```
+will evaluate to `false`.
+
 When the argument types of an operator/function invocation don't match the signature of the operator/function being invoked, the CESQL engine MUST try to perform an implicit cast.
 
 This section defines an **ambiguous** operator/function as an operator/function that is overloaded with another