The seq function is the most basic method of introducing strictness to a Haskll program. seq :: a -> b -> b takes two arguments of any type, and returns the second. However, it also has the important property that it is magically strict in its first argument. In essence, seq is defined by the following two equations:
⊥ `seq` b = ⊥ a `seq` b = b
A common misconception regarding seq is that seq x "evaluates" x. Well, sort of. seq doesn't evaluate anything just by virtue of existing in the source file. All it does is introduce an artificial dependency of one value on another: when the result of seq is evaluated, the first argument must also be evaluated. As an example, suppose x :: Integer, then seq x b is essentially a bit like if x == 0 then b else b - unconditionally equal to b, but forcing x along the way. In particular, the expression x `seq` x is completely redundant, and always has exactly the same effect as just writing x.
Strictly speaking, the two equations of
seq are all it must satisfy, and if the compiler can statically prove that the first argument is not ⊥, it doesn't have to evaluate it to meet its obligations. In practice, this almost never happens, and would probably be considered highly counterintuitive behaviour on the part of GHC (or whatever else you use to run your code). However, it is the case that evaluating
b and then
a, then returning
b is a perfectly legitimate thing to do; it is to prevent this ambiguity that
pseq was invented, but that's another story.
seq is the only way to force evaluation of a value with a function type.