Debugging

io:format

The io:format function prints with a format string. The tilde p prints any term readably, and the tilde n ends the line.

The second argument is the list of values to print, in order.

1> io:format("name: ~p, age: ~p~n", ["Ada", 36]).

name: "Ada", age: 36

ok

erlang:display

The erlang:display function writes one term and answers the atom ok. It prints to the group leader even when the shell is busy.

Use it inside a crashy loop where io:format might itself get tangled.

1> erlang:display({point, 1, 2}).

{point,1,2}

ok

The dbg tracer

The dbg module traces calls without editing code. Start the tracer, set a pattern, and watch matching calls arrive.

This example traces every call to a module named mymod.

1> dbg:tracer().

ok,<0.91.0>

2> dbg:p(all, c).

ok,[{matched,nonode@nohost,_}]}

3> dbg:tpl(mymod, x, [return_trace]).

Shell habits

The shell keeps a numbered history. The b function prints every binding, and f forgets them all, which is the fastest reset when a name gets stuck.

For deep problems, reproduce the failure in a fresh shell first: half of all bugs hide in stale bindings.

1> b().

Name = "Ada"

2> f().

ok

Next: Encodings and Unicode

Article author: Arthur Isaev