To be clear (for readers, I know parent knows this), the return values of the effects are discarded. (The first sentence maybe should say "discard its results", not "discard it's effects").
The point is to make the (side-)effects happen (since non-strictness means the effects have to be forced, by calling `(>>=)` or similar) but not use the return value (which is how computations are usually forced in Haskell)
And also, the effects happen in the order determined by the function forM_ or whoever, instead of in a hard-to-impossible-to-predict order of the pure non-monadic/non-effect computations defined by equations of function applications.
(The M in fooM is for Monad, indicating that bind (f <<= x) is used instead of simple function application (f x) that is used in the "cousin" function foo. But that is just naming convention.)
The point is to make the (side-)effects happen (since non-strictness means the effects have to be forced, by calling `(>>=)` or similar) but not use the return value (which is how computations are usually forced in Haskell)
And also, the effects happen in the order determined by the function forM_ or whoever, instead of in a hard-to-impossible-to-predict order of the pure non-monadic/non-effect computations defined by equations of function applications.
(The M in fooM is for Monad, indicating that bind (f <<= x) is used instead of simple function application (f x) that is used in the "cousin" function foo. But that is just naming convention.)