Confusing case_clause exception instead of function_clause
#5513
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bjorng
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Jan 13, 2022
Consider this module:
-module(bug).
-export([foo/0]).
foo() ->
fun(a) -> ok end(b).
The call to the fun will always fail, which will be noted by
the compiler:
1> c(bug).
bug.erl:5:5: Warning: no clause will ever match
% 5| fun(a) -> ok end(b).
% | ^
{ok,bug}
What is unexpected is that the exception that is raised is not
a `function_clause` exception:
2> bug:foo().
** exception error: no case clause matching {b}
in function bug:foo/0 (bug.erl, line 5)
This confusing `case_clause` exception started to appear in OTP 24
because of 72675ba (erlang#4545) that inlines funs that are
immediately used.
Before OTP 24, when the fun was not inlined, the exception would be:
2> bug:foo().
** exception error: no function clause matching bug:'-foo/0-fun-0-'(b) (bug.erl, line 5)
The reason that `function_clause` exceptions are rewritten to
`case_clause` exceptions in inlined code is to avoid the even more
confusing and misleading exception:
2> bug:foo().
** exception error: no function clause matching bug:foo(b) (bug.erl, line 5)
This is confusing because it seems that `foo/0` was called with one argument.
To reduce the confusion, this commmit ensures that `function_clause` exceptions in
inlined code remains `function_clause` exceptions, but with a generated name that
makes it clear that the `function_clause` exception occurred in a fun:
2> bug:foo().
** exception error: no function clause matching bug:'-foo/0-inlined-0-'(b) (bug.erl, line 5)
Fixes erlang#5513
bjorng
added a commit
to bjorng/otp
that referenced
this issue
Jan 14, 2022
Consider this module:
-module(bug).
-export([foo/0]).
foo() ->
fun(a) -> ok end(b).
The call to the fun will always fail, which will be noted by
the compiler:
1> c(bug).
bug.erl:5:5: Warning: no clause will ever match
% 5| fun(a) -> ok end(b).
% | ^
{ok,bug}
What is unexpected is that the exception that is raised is not
a `function_clause` exception:
2> bug:foo().
** exception error: no case clause matching {b}
in function bug:foo/0 (bug.erl, line 5)
This confusing `case_clause` exception started to appear in OTP 24
because of 72675ba (erlang#4545) that inlines funs that are
immediately used.
Before OTP 24, when the fun was not inlined, the exception would be:
2> bug:foo().
** exception error: no function clause matching bug:'-foo/0-fun-0-'(b) (bug.erl, line 5)
The reason that `function_clause` exceptions are rewritten to
`case_clause` exceptions in inlined code is to avoid the even more
confusing and misleading exception:
2> bug:foo().
** exception error: no function clause matching bug:foo(b) (bug.erl, line 5)
This is confusing because it seems that `foo/0` was called with one argument.
To reduce the confusion, this commmit ensures that `function_clause` exceptions in
inlined code remains `function_clause` exceptions, but with a generated name that
makes it clear that the `function_clause` exception occurred in a fun:
2> bug:foo().
** exception error: no function clause matching bug:'-foo/0-inlined-0-'(b) (bug.erl, line 5)
Fixes erlang#5513
bjorng
added a commit
that referenced
this issue
Jan 20, 2022
…-17860 Eliminate confusing `case_clause` exception
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Describe the bug
When a fun is used immediately, the compiler will inline it, and if no clause in the fun matches, there will be a
case_clauseerror instead of afunction_clauseerror.To Reproduce
Consider this module:
Compile and run it:
Expected behavior
Affected versions
OTP 24, which introduced automatic inlining of funs that are immediately used and discarded.
Additional context
While not strictly a bug, this behavior is confusing.
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