What is a monad?

Its a concept that was invented in mathematics  in the 1960s, and it was rediscovered in computer science in the 1990s

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In functional programming, a monad is a software design pattern with a structure that combines program fragments (functions) and wraps their return values in a type with additional computation

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So

(x, y) → x + y

can be curried into

x → (y → x + y)

 

Or, in classical notation

f(x, y) = x + y

can be curried into

g(x) = y → f(x, y)

curry

Monads in pictures