Underscore Variables
There’s one more thing to say about variables. The special syntax
_VarName is used for a normal variable, not an anonymous
variable. Normally the compiler will generate a warning if a
variable is used only once in a clause since this is usually the
sign of an error. If the variable is used only once but starts
with an underscore, the warning message will not be generated.
Since _Var is a normal variable, very subtle bugs can be
caused by forgetting this and using it as a “don’t
care” pattern. In a complicated pattern match, it
can be difficult to spot that, for
example, _Int is repeated when it shouldn’t have been,
causing the pattern match to fail.
There are two main uses of underscore variables.
-
To name a variable ...
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