concept seq in category haskell

This is an excerpt from Manning's book Haskell in Depth MEAP V12. Login to get full access to this book.

The seq function

Haskell provides somewhat magic seq function, which is defined in terms of strictness. Its type is a → b → b so it looks like it ignores the first argument. In fact, it’s not, because its implementation is guaranteed to adhere to the following two rules:

ghci> let _ = undefined in "OK"
"OK"
ghci> :set -XBangPatterns
ghci> let !_ = "BAD" in "OK"
"OK"
ghci> let !_ = undefined in "OK"
"*** Exception: Prelude.undefined

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