Function Repository Resource:

NoEntry

Hold an expression and make its content inaccessible to other functions

Contributed by: Sjoerd Smit

ResourceFunction["NoEntry"][expr]

returns a Hold object that maintains expr in an unevaluated form and prevents access even from functions like ReplaceAll and Part.

Examples

Basic Examples (2)

NoEntry returns a special Hold object:

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NoEntry prevents its contents from being modified by ReplaceAll, unlike normal Hold:

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${1, ResourceFunction["NoEntry"][2], Hold[3], "a"} /. _Integer -> "x"$

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NoEntry objects cannot be accessed by Part:

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Remove the Hold wrapper again to access the contents:

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The FullForm is the same as a regular Hold object:

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Scope (1)

Use Evaluate to evaluate the contents of NoEntry before making them inaccessible:

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Applications (5)

NoEntry can be used in ragged expressions to make it easy to find the lowest-level elements if those elements still have some internal structure. Consider the following linked list of symbolic numbers:

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$lst1 = Fold[List, {1, Sqrt[2], 3^Pi, Log[5], E + Pi}]$

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Mapping a function to the numbers inside the linked list can be difficult because the {-1} level specification doesn't correspond to the list elements:

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NoEntry hides the internal structure of the elements:

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$lst2 = Fold[List, ResourceFunction["NoEntry"] /@ {1, Sqrt[2], 3^Pi, Log[5], E + Pi}]$

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Map the function to the numbers:

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$Map[Replace[#, Hold[e_] :> f[e]] &, lst2, {-1}]$

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Use a Listable Function to apply NoEntry at the bottom of ragged arrays:

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Properties and Relations (2)

NoEntry objects are atoms:

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The contents of NoEntry objects are accessible by pattern matching the whole Hold object:

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Possible Issues (2)

ReleaseHold doesn't work on objects created by NoEntry:

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NoEntry needs to evaluate before it becomes inaccessible, so it doesn't work directly when placed inside another inert wrapper like Hold:

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$Hold[{ResourceFunction["NoEntry"][1], ResourceFunction["NoEntry"][2]}] /. _Integer -> "x"$

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The following replacement rule can be used to force all NoEntry objects to evaluate inside an expression:

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$expr = ReplaceAll[ Hold[{ResourceFunction["NoEntry"][1], ResourceFunction["NoEntry"][2]}], e_ /; Head[Unevaluated@e] === ResourceFunction["NoEntry"] :> With[{eval = e}, eval /; True ] ]$