Thursday, December 22, 2005

Expressions and statements

This post explaining the I/O monad to a newbie on the Haskell mailing list motivates it in terms of expressions and statements:
We have a[n] "evaluation machine" and a[n] "execution machine". The former
evaluates expressions, the latter executes I/O actions. When the program is
started, the execution machine wants to execute the actions from the "main to
do list".
Interesting. Imperative languages like C distinguish expressions and statements syntactically, but with various implicit conversions between them, e.g., expression-statements and procedures that execute statements but then return values. Haskell separates statements from expressions with the type system, and keeps them apart more strictly (no pun intended). But in a sense they're both making the same distinction.

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