3.1 Specifying syntactic structure w/ rules

• Backus-Naur Form (BNF) a rule notation for presenting the syntax of languages
  • the BNF specification for noun phrases:

⟨nounphrase⟩∶∶= ⟨adjective⟩⟨nounphrase⟩
⟨nounphrase⟩∶∶= ⟨nounphrase⟩⟨noun⟩

• where ⟨ is considered a class, ⟨noun⟩ is considered the class of noun expressions, and ∶∶= is considered “can be composed from”

⟨nounphrase⟩∶∶= ⟨noun⟩
∣ ⟨adjective⟩⟨nounphrase⟩
∣ ⟨nounphrase⟩⟨noun⟩

• The BNF notation allows separating alternative right-hand sides with the vertical bar (∣) as we have done here
• semantics the realm of characterization of the meanings of expressions on the basis of their structure

3.2 Disambiguating ambiguous expressions

• we use conventions and annotations to decrease operational ambiguity
• order of operations, precedence, associativity of an operator - left associative when the operations are applied starting with the left one, right associative ie. OCaml’s exponentiation operator **
  • 2. ** 2. ** 3. is disambiguated as 2. ** (2. ** 3.). Its value is 256., not 64

3.3 Abstract and concrete syntax

abstract syntax for expressions qua structured objects

concrete syntax for their linear-notated manifestations

ie. (3+4)+5

<aside> 💡 Note: concrete syntax structures may produce certain expressions that seem related but that don’t have the same abstract syntax. For example, 5 * (3 + 4) wouldn’t have the same syntax as (3+4)+5

</aside>

• ~-, the unary negation operator

3.4 Expression your intentions

• In OCaml, comments are marked by surrounding them with special delimiters: (* ⟨⟩ *)
  • comments should describe the why rather than the how