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Asked: 2026-05-18T05:40:49+00:00

I often need to debug by preventing some definition from evaluating and checking intermediate

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I often need to debug by preventing some definition from evaluating and checking intermediate results. I accomplish this by doing initAll;clearAll[f,g,h]. I don’t like it because

  1. It forces you to put everything in a single init block
  2. It’s not flexible enough to only keep certain patterns like f[1] unevaluated

Instead I’d like to have a list forbidden patterns and have any pattern that matches left unevaluated. How can I achieve this?

Edit
So far I found this pattern the most useful (it’s Michael Pilat’s answer except with HoldForm and BlankNullSequence)

eh[expr_, symbols : {___Symbol}] := Block[symbols, HoldForm@Evaluate[expr]]

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1 Answer

  1. Editorial Team

    Block can help with what you want:

    f[x_] := x + 1;
g[x_] := x - 1;

In[13]:= Block[{f},
 Hold@Evaluate[(f[g[a]]^2)]
 ]

Out[13]= Hold[f[-1 + a]^2]

    Do you want to prevent evaluation for certain down-value patterns of f? (E.g., block f[x_] but allow f[x_, y_])?

    UPDATE

    Here’s a functional form:

    SetAttributes[EvaluateHeld, HoldAll];
EvaluateHeld[expr_, symbols : {__Symbol}] :=
  Block[symbols, Hold@Evaluate[expr]
]

In[7]:= EvaluateHeld[f[g[a]]^2, {f}]

Out[7]= Hold[f[-1 + a]^2]
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