This paper presents a new model for performing input/output (I/O) in a purely functional language, based on monads. The approach allows for composable, extensible, and efficient I/O operations without requiring changes to the type system. It extends smoothly to support mixed-language programming and in-place array updates. The paper introduces a type IO a, which represents actions that may perform I/O and then return a value of type a. Actions are composed using combinators such as seqIO and bindIO, which allow for the sequential execution of I/O operations. The approach is compared with other I/O styles, including dialogues and continuations, and is shown to be as expressive as these approaches. The monadic approach is implemented in Haskell, with the ability to call C procedures directly via the ccall construct. This allows for efficient I/O operations, as the Haskell compiler can generate C code that closely resembles hand-written code. The approach also supports incremental arrays with in-place updates, and is based solely on the Hindley-Milner type system, without requiring linear or existential types. The paper also discusses the extension of the monadic approach to support reference types and arrays, and compares it with continuation-based approaches. It is shown that monads can implement continuations, but not vice versa. The approach is efficient and expressive, and is suitable for both pure and mixed-language programming. The paper concludes that the monadic approach is a powerful and flexible way to implement I/O in a purely functional language.This paper presents a new model for performing input/output (I/O) in a purely functional language, based on monads. The approach allows for composable, extensible, and efficient I/O operations without requiring changes to the type system. It extends smoothly to support mixed-language programming and in-place array updates. The paper introduces a type IO a, which represents actions that may perform I/O and then return a value of type a. Actions are composed using combinators such as seqIO and bindIO, which allow for the sequential execution of I/O operations. The approach is compared with other I/O styles, including dialogues and continuations, and is shown to be as expressive as these approaches. The monadic approach is implemented in Haskell, with the ability to call C procedures directly via the ccall construct. This allows for efficient I/O operations, as the Haskell compiler can generate C code that closely resembles hand-written code. The approach also supports incremental arrays with in-place updates, and is based solely on the Hindley-Milner type system, without requiring linear or existential types. The paper also discusses the extension of the monadic approach to support reference types and arrays, and compares it with continuation-based approaches. It is shown that monads can implement continuations, but not vice versa. The approach is efficient and expressive, and is suitable for both pure and mixed-language programming. The paper concludes that the monadic approach is a powerful and flexible way to implement I/O in a purely functional language.