Reference Guides » Undocumented Contexts

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System*

The "System`*`" contexts provide similar functionality to the "Internal`*`" context in terms of low-level usage.

There are a huge number of these contexts, so we'll just focus on a few of them.

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SystemPrivate

SetNoEntry

This is a special function that tags an expression as being treated as atomic (i.e. it cannot be "entered" by Part )

See here

NoEntryQ

This tests whether an expression has had SetNoEntry called on it

EntryQ

Opposite of NoEntryQ

HoldSetNoEntry

SetNoEntry but with held argument

HoldEntryQ

EntryQ but with held argument

ConstructNoEntry

Constructs an atomic object, rather than having this set after the fact

Arguments

Collects the arguments to an expression and checks that the number of them is within an expected range. It holds its first argument.

  • Single Number
 System`Private`Arguments[test[1, 2, 3], 3]

 {{1,2,3},{}}
  • Variable Number
 System`Private`Arguments[test[1, 2], {2, 3}]

 {{1,2},{}}
  • Different Head
 System`Private`Arguments[test[1, 2, 3], 3, Hold]

 {Hold[1,2,3],Hold[]}
  • With Options
 Options[test]={"Option1"->1, "Option2"->2};
System`Private`Arguments[test[1, 2, 3, "Option1"->2], 3, Hold]

 {Hold[1,2,3],Hold["Option1"->2]}
  • Bad option
 Options[test]={"Option1"->1, "Option2"->2};
System`Private`Arguments[test[1, 2, 3, "Option"->2], 3, Hold]
test::optx: Unknown option "Option" in test[1,2,3,"Option"->2]. 
 {}
  • Incorrect Number
 System`Private`Arguments[test[1, 2, 3], 5]
test::argrx: test called with 3 arguments; 5 arguments are expected. 
 {}

ArgumentsWithRules

Like Arguments but reports that it expects to have options if too many non-option arguments are passed: 

 System`Private`ArgumentsWithRules[test[1, 2, 3, 4, "Option1"->4], 3]
test::nonopt: Options expected (instead of 4) beyond position 3 in test[1,2,3,4,"Option1"->4]. An option must be a rule or a list of rules. 
 {}
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SystemUtilities

These tend to be a bit more developer oriented than many of the "System`Private`" functions and a bit higher-level.

SymbolList

Extracts the list of symbols in an expression. Doesn't hold its argument.

  • All symbols
 System`Utilities`SymbolList[a[b[c[d]]]]

 {d,b,a,c}
  • Wrapped in a Head
 System`Utilities`SymbolList[a[b[c[d]]], Hold]

 {Hold[d],Hold[b],Hold[a],Hold[c]}
  • Ignored contexts
 System`Utilities`SymbolList[Hold@Print@Hold@Print@a[b[c[d]]], Hold, {"System`"}]

 {Hold[d],Hold[b],Hold[a],Hold[c]}

HashTable

Makes a new HashTable object. Less useful since version 10 with the introduction of Association .

See here

HashTableAdd

HashTableGet

HashTableRemove

Bits of the HashTable API