$Pre

$Pre
is a global variable whose value, if set, is applied to every input expression.

DetailsDetails

  • Unless $Pre is assigned to be a function which holds its arguments unevaluated, input expressions will be evaluated before $Pre is applied, so the effect of $Pre will be the same as $Post.
  • $Pre is applied to expressions, while $PreRead is applied to strings which have not yet been parsed into expressions.

ExamplesExamplesopen allclose all

Basic Examples  (2)Basic Examples  (2)

By default, $Pre is not defined, meaning the input is evaluated directly:

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Save input expressions in held form:

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Evaluation works as expected:

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Restore $Pre to its default:

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The variable holds the inputs in unevaluated form:

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Introduced in 1988
(1.0)