$PrePrint¶

Status: Stable

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

Description¶

$PrePrint
    is a global variable whose value, if set, is applied to every
    expression just before it is printed. Out[n] is assigned the
    unmodified result, but the printed form reflects the value
    returned by $PrePrint.

Examples¶

No verified examples yet for this function.

Implementation notes¶

A REPL session hook, not a builtin. Registered (docstring only) in repl_hooks_init (src/repl_hooks.c). repl.c calls repl_apply_pre_print(out) just before printing; if an OwnValue is set, hook_call_eval builds and evaluates $PrePrint[expr]. Crucially this is display-only: Out[n] is assigned the unmodified post-$Post result above, and only the rendered form reflects the $PrePrint value. Unset = identity.

Attributes: none registered.

Implementation status¶

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

References¶

Source: src/repl_hooks.c
Specification: docs/spec/builtins/control-flow.md

Notes & additional examples¶

Worked examples¶

In[1]:= $PrePrint = Framed
Out[1]= Framed[Framed]

In[2]:= 3 + 4
Out[2]= Framed[7]

Notes¶

$PrePrint, if set, is applied to every expression just before it is printed. The displayed form reflects the $PrePrint value (here wrapped in Framed), but Out[n] is assigned the unmodified result, so later references see the plain value. Unset by default.