Easy printing of executable code

Question

I'm working on a package that has functions that call other existing and packaged functions. I'd like a way to output what some of those key calls are, so users can see what's happening under the hood and for debugging purposes.

Here's a minimal example that wraps around NDSolve:

ex[eqns_, unks_, tmax_] := (
  Print["NDSolve[", eqns, ", ", unks, ", {t, 0, ", tmax, "}]"]; (* output code *)
  NDSolve[eqns, unks, {t, 0, tmax}]
)

ex[{n'[t] == n[t] (1 - n[t]), n[0] == 0.01}, {n}, 10]

NDSolve[{n'[t]==(1-n[t]) n[t],n[0]==0.01}, {n}, {t, 0, 10}]

{{n->InterpolatingFunction[...]}}

This approach basically works but has two issues I'd like to fix:

Issue 1: The output code is ugly and a minor hassle to write correctly.

Issue 2: If you copy and paste the output cell, it comes out as

"NDSolve["{n'[t]==(1-n[t]) n[t],n[0]==0.01`}", "{n}", {t, 0, "10"}]"

which isn't executable. I'd want to get:

NDSolve[{n'[t]==(1-n[t]) n[t],n[0]==0.01}, {n}, {t, 0, 10}]

Any ideas on how to address either (or hopefully both) of these issues?

Answer 1

You could define a function similar to Echo with the difference that it won't evaluate its argument before printing.

SetAttributes[echo, HoldAll]

echo[expr_] := (CellPrint@
   ExpressionCell[Defer[expr], CellDingbat -> ">"];
   expr)

Then just put it in front of anything you want printed:

ex[eqns_, unks_, tmax_] := echo@NDSolve[eqns, unks, {t, 0, tmax}]

Let's try it:

ex[{n'[t] == n[t] (1 - n[t]), n[0] == 0.01}, {n}, 10]

The output is copyable.

---

Print should work fine instead of CellPrint/ExpressionCell. I used CellPrint so I would be able to add a CellDingbat.

---

Caveat: If eqns, unks, etc. were not directly substituted (e.g. if they were Module or Block variables) then their name would be printed instead of their value. Use With to directly substitute any value you want to print. For example,

In[20]:= fun[] := Module[{x = 2},
  echo[x^2]
  ]

In[21]:= fun[]

> x$21572^2

Out[21]= 4

In[22]:= fun[] := Module[{x = 2},
  With[{x = x}, echo[x^2]]
  ]

In[23]:= fun[]

> 2^2

Out[23]= 4

---

If you want to turn off debugging, simply set ClearAll[echo]; echo[x_] := x.