a monad is a type that is basically a structure { X:value-that-we-care-about, Y:something-we-only-care-about-occasionally-if-at-all } and then a bun

a monad is a type that is basically a structure { X:value-that-we-care-about, Y:something-we-only-care-about-occasionally-if-at-all } and then a bunch of functions that let you pretend it's the value X, that's what the compiler is abbreviating for you, all the necessary wrapping/unwrapping or calling members. In the prolog example what we care about is the last/latest value and monads let us just shove the previous value(s) into a hole and forget about it, or at least ignore it until later. with error monads we don't always care if something failed (like a write) but we probably want to keep that information for later, so it just gets hidden and we can pretend we're just passing around a value (see https://fsharpforfunandprofit.com/rop/ https://fsharpforfunandprofit.com/rop/ ) I'm not particularly clear on how Haskell does the IO monad, but basically the hidden value is what lets the compiler keep track of what was done pre/post IO, allowing you to thread-the-whole-world through an IO call. "Do" uses the same trick as that prolog example, by threading previous calculations through a function call you can force Do Foo1 Foo2 Foo3 to be called in that order (important for a lazy language) so you don't have force it by nesting them Foo3 (Foo2(Foo1(X))) can get pretty ugly fast.