How to see abstract syntax tree of Wolfram Language expression

Question

I mean, suppose I have this expression:

2 + 2

I want to get something like this:

Plus[2, 2]

as an output. I've tried FullForm, but this just evaluates 2+2 and returns 4. Is this even possible?

You can try prepending a Hold. Hold[2+2] // FullForm. You might also like TreeForm for this purpose. — evanb, Aug 27 '14 at 21:45
Generally, what you are seeking is to prevent kernel evaluation. Hold and its ilk (Defer, Unevaluated, Inactive, etc.) are the mechanisms for this, depending on precisely what you want. Look at this page for more information. — mfvonh, Aug 27 '14 at 21:49

Community · Accepted Answer · 2017-04-13 12:56:03Z

tree = Function[x, Defer @ FullForm @ x, HoldAll];

Now:

2 + 2 // tree

Plus[2, 2]

I used Defer to allow the output to be evaluated. If you do not prefer this replace it with HoldForm.

For some explanation of the mechanics of this code see: Why doesn't "Defer" work with "TableForm"?

See also my standard methods for analyzing parsing.

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