I/O is among the impure features of ML since I/O in ML involves side effect.
B.12.1 Input
The option data type has two values: NONE and SOME. Use isSome (χ) to determine the value of a variable of type option. Use valOf (χ) to extract the value of a variable of type option. A string option list is not the same as a string list.
Standard Input
The standard input stream generally does not need to be opened and closed.
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