Curry: An Integrated Functional Logic Language

Now the names f, m1.f, and m2.f refer to the identical entity, namely the function f defined
in module m. Note that there is no need to qualify f in the module main since this name is
unambiguously resolved to the function f defined in module m, although it is imported via two
different paths.
Qualified names are treated syntactically like unqualified names. In particular, a qualified infix
operator like Complex.+ has the same fixity as the definition of + in the module Complex, i.e., the
expression “x Complex.+ y” is syntactically valid. To distinguish between a function composition
like “f . g”, where “.” is an infix operator (see Section 14, page 46), and a module qualification,
spaces are not allowed in the module qualification while there must be at least one space before
the “.” if it is used as an infix operator. Thus, “f . g” is interpreted as the composition of f and
g whereas “f.g” is interpreted as the object g imported from module f.
The import dependencies between modules must be non-circular, i.e., it is not allowed that
module m1 imports module m2 and module m2 also imports (directly or indirectly) module m1.
7 Input/Output
Curry provides a declarative model of I/O by considering I/O operations as transformations on
the outside world. In order to avoid dealing with different versions of the outside world, it must
be ensured that at each point of a computation only one version of the world is accessible. This is
guaranteed by using the monadic I/O approach [42] of Haskell and by requiring that I/O operations
are not allowed in program parts where non-deterministic search is possible.
7.1 Monadic I/O
In the monadic I/O approach, the outside “world” is not directly accessible but only through actions
which change the world. Thus, the world is encapsulated in an abstract datatype which provides
actions to change the world. The type of such actions is IO t which is an abbreviation for
World -> (t,World)
where World denotes the type of all states of the outside world. If an action of type IO t is applied
to a particular world, it yields a value of type t and a new (changed) world. For instance, getChar
is an action which reads a character from the standard input when it is executed, i.e., applied to
a world. Therefore, getChar has the type IO Char. The important point is that values of type
World are not accessible for the programmer — she/he can only create and compose actions on
the world. Thus, a program intended to perform I/O operations has a sequence of actions as the
result. These actions are computed and executed when the program is connected to the world by
executing it. For instance,
getChar :: IO Char
getLine :: IO String
are actions which read the next character or the next line from the standard input. The functions
putChar :: Char -> IO ()
putStr :: String -> IO ()
putStrLn :: String -> IO ()
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