turn off lazy evaluation in haskell

Question

Is it possible to turn off lazy evaluation in Haskell?

Is there a specific compiler flag of library to facilitate this?

I wanted to try something new with a old program that I had written a while back, to see if I can improve performance.

Answer 1

There are a number of ways to turn a lazy thing strict. You can:

1. Explicitly insert a spurious pattern match.
2. Use seq or its close relative ($!).
3. Use BangPatterns.
4. Use strictness annotations on your types.

More information here.

Answer 2

You can't turn off laziness, because the I/O system of Haskell depends on it. Without lazy evaluation this program would run into a busy loop without ever outputting anything:

main = forever (putStrLn "Hello!")

This is because forever c is an infinite program. With lazy evaluation the program is calculated only as far as necessary to run the next instruction. If you turn off laziness, every function becomes strict, including (>>), which basically makes the forever function diverge:

forever c = let cs = c >> cs in cs

However, you can add strictness annotations to constructors and patterns. When a function is strict, its argument is forced as part of the evaluation of the result independent of whether the argument is needed or not. This resembles eager evaluation.