CSCE 314 Lecture 23

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Type Checking

Context-free grammars cannot catch type errors (e.g. whether the conditional expression within an if-statement will evaluate to a Boolean).

CF grammars cannot tie a variable to its definition. Type-checker makes sure that the AST nodes' types match what the language/program requires.

Assigning types:

• We say "expression t is of type T"
• Notation is
  • $t:T$
  • $t\in T$
  • $\Gamma \vdash t:T$, where Γ is the context or typing environment
  • Haskell uses t :: T