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$Pre

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

$Pre

is a global variable whose value, if set, is applied to every input expression after parsing and before standard evaluation.

Notes Unless $Pre is assigned to a head with HoldAll, the wrapped expression is evaluated before $Pre sees it -- which makes the effect indistinguishable from $Post.

Examples (2)

Every input below was run against the current Mathilda build and its output recorded.

Applications (2)

In[1]:= $Pre = Hold
Out[1]= Hold

In[2]:= 1 + 1
Out[2]= Hold[1 + 1]

Implementation notes

A REPL session hook, not a builtin. repl_hooks_init (src/repl_hooks.c) merely touches the symbol so ?$Pre works; no default OwnValue is installed, so out of the box the hook is a no-op. Each REPL iteration repl.c calls repl_apply_pre(parsed); if hook_is_set("$Pre") (i.e. symtab_get_own_values("$Pre") is non-empty) it builds $Pre[expr] and runs it through the standard evaluate() via hook_call_eval, applied after parsing but before the main evaluation. Because the wrapped expression is evaluated before $Pre sees it unless $Pre is assigned a HoldAll head, its effect is usually indistinguishable from $Post.

Attributes: none registered.

References

Notes & additional examples

Notes

$Pre, if set, is applied to every input expression after parsing and before standard evaluation. Using a head with HoldAll (such as Hold) lets $Pre intercept the unevaluated input; otherwise the argument is evaluated first and the effect is indistinguishable from $Post. Unset by default.