What is the difference between a monad and a closure?

Question

i am kinda confused reading the definition between the two. Can they actually intersect in terms of definition? or am i completely lost? Thanks.

Answer 1

Closures, as the word tends to be used, are just functions (or blocks of code, if you like) that you can treat like a piece of data and pass to other functions, etc. (the "closed" bit is that wherever you eventually call it, it behaves just as it would if you called it where it was originally defined). A monad is (roughly) more like a context in which functions can be chained together sequentially, and controls how data is passed from one function to the next.

Answer 2

They're quite different, although monads will often use closures to capture logic.

Personally I would try to get solid on the definition of closures (essentially a piece of logic which also captures its environment, i.e. local variables etc) before worrying about monads. They can come later :)

There are various questions about closures on Stack Overflow - the best one to help you will depend on what platform you're working on. For instance, there's:

• What are closures in .NET?
• Function pointers, closures and lambda

Personally I'm only just beginning to "grok" monads (thanks to the book I'm helping out on). One day I'll get round to writing an article about them, when I feel I understand them well enough :)

Answer 3

A "closure" is an object comprising 1) a function, and 2) the values of its free variables where it's constructed.

A "monad" is a class of functions that can be composed in a certain way, i.e. by using associated bind and return higher-order function operators, to produce other functions.

Answer 4

I think monads are a little more complicated than closures because closures are just blocks of code that remember something from the point of their definitions and monads are a construct for "twisting" the usual function composition operation.