18b Logical operators

[ngjunsiang]

Now that our interpreter can understand TRUE and FALSE, let's implement AND, OR, and NOT operators.

We start with scanning. First let's add the operators in builtin.py and have them recognised as operators:
https://github.com/nyjc-computing/pseudo/blob/4241f231018f605311a67ad762c9db3e4cf5e8cb/builtin.py#L71-L78
https://github.com/nyjc-computing/pseudo/blob/4241f231018f605311a67ad762c9db3e4cf5e8cb/builtin.py#L105-L119

Then enable scanning them:
https://github.com/nyjc-computing/pseudo/blob/4241f231018f605311a67ad762c9db3e4cf5e8cb/scanner.py#L98-L100

[ngjunsiang]

Operator precedence

NOT has the same precedence as unary -, so we parse it at the same line:
https://github.com/nyjc-computing/pseudo/blob/96d886e011e1e6c384f6c56d2546a9d81b7000cd/parser.py#L123-L124

AND and OR have lower precedence than <> and =; they have the lowest precedence of all the operators. So we add a new parser for them, between equality() and expression():
https://github.com/nyjc-computing/pseudo/blob/96d886e011e1e6c384f6c56d2546a9d81b7000cd/parser.py#L193-L207

[ngjunsiang]

Resolving logical operators

Next, we've got to resolve these new operators so that appropriate types are returned. First, the unary NOT:
https://github.com/nyjc-computing/pseudo/blob/160a9282da65ecaf1dc841f3284d7aa922f311c9/resolver.py#L52-L54

Then the binary AND & OR:
https://github.com/nyjc-computing/pseudo/blob/4deb5a00d98af85be24d26fc5e89ebc028e8549b/resolver.py#L59-L62

[ngjunsiang]

Interpreting logical operators

Okay, nothing to change here; our operators take in the )type-checked) input, and return their output. But a slightly horrifying discovery:
https://github.com/nyjc-computing/pseudo/blob/4deb5a00d98af85be24d26fc5e89ebc028e8549b/interpreter.py#L29-L31

For unary -, we are passing a single argument to our sub operator, which only accepts two arguments 😱:
https://github.com/nyjc-computing/pseudo/blob/4deb5a00d98af85be24d26fc5e89ebc028e8549b/builtin.py#L44-L45

Fix - we overload sub to also handle a single argument, by returning its negative:
https://github.com/nyjc-computing/pseudo/blob/763213fc914a2b1cf2a44a2f64a21bc046432343/builtin.py#L44-L47

[ngjunsiang]

And we have working logical operators 👍