WOLFRAM

Wolfram Language & System Documentation Center

"HeldExpression" (Interpreter Type)

Wolfram Language expression.

Details

Allowed Forms

  • Any valid Wolfram Language expression.

Restriction Specifications

Interpretation

Examples

Basic Examples  (5)

Interpret expressions and avoid evaluation:

Wolfram Language code: Interpreter["HeldExpression"]["1/2 + 3/5 - 2/3"]
Wolfram Language code: Interpreter["HeldExpression"]["Cos[Pi]"]

No evaluation is attempted when matching the choices:

Wolfram Language code: Interpreter[Restricted["HeldExpression", {{1, 2, 3, 4}}]]["2+1"]
Wolfram Language code: Interpreter[Restricted["HeldExpression", {{2, HoldComplete[2 + 1]}}]]["2+1"]

By default, only expressions with no side effect are allowed:

Wolfram Language code: Interpreter["HeldExpression"]["Quit[]"]

This can be changed by allowing All symbols:

Wolfram Language code: Interpreter[Restricted["HeldExpression", All]]["Quit[]"]

Restrict to a list of symbols:

Wolfram Language code: Interpreter[Restricted["HeldExpression", {Map, List, Function, Slot}]]["#&/@{1,2,3}"]
Wolfram Language code: Interpreter[Restricted["HeldExpression", {Map, List, Function, Slot}]]["#+1&/@{1,2,3}"]

By default, new symbols are created in $Context, but a custom context can be specified:

Wolfram Language code: Interpreter[Restricted["HeldExpression", Automatic, Automatic, Automatic]]["x"] Context[x]
Wolfram Language code: Interpreter[Restricted["HeldExpression", Automatic, Automatic, "mycontext`"]]["y"] Context[y]