05-02 The IO Type (Introduction to Haskell) 

By introducing an abstract IO type for IO actions or plans, we solve the problem. Evaluating IO actions never executes any side effects. In other words, one can evaluate a plan without putting it into action. Execution instead happens via either making an IO action part of the main entry point to a program, or alternatively, by asking GHCi to execute an IO action for us.
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