Crash in Gradient.ElixirExpr.pp_guards

[lukaszsamson]

some_module.ex: ** (FunctionClauseError) no function clause matching in Gradient.ElixirExpr.pp_guards/1    
    
    The following arguments were given to Gradient.ElixirExpr.pp_guards/1:
    
        # 1
        [[{:call, [generated: true, location: 21], {:remote, [generated: true, location: 21], {:atom, [generated: true, location: 21], :erlang}, {:atom, [generated: true, location: 21], :is_atom}}, [{:var, [generated: true, location: 21], :_@1}]}, {:op, [generated: true, location: 21], :"=/=", {:var, [generated: true, location: 21], :_@1}, {:atom, [generated: true, location: 21], nil}}, {:op, [generated: true, location: 21], :"=/=", {:var, [generated: true, location: 21], :_@1}, {:atom, [generated: true, location: 21], true}}, {:op, [generated: true, location: 21], :"=/=", {:var, [generated: true, location: 21], :_@1}, {:atom, [generated: true, location: 21], false}}]]
    
    Attempted function clauses (showing 2 out of 2):
    
        def pp_guards([])
        def pp_guards([[guard]])
    
    (gradient 0.1.0) Gradient.ElixirExpr.pp_guards/1
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:310: Gradient.ElixirExpr.pp_case_clause/1
    (elixir 1.14.2) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2
    (elixir 1.14.2) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:248: Gradient.ElixirExpr.pp_clauses/2
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:207: Gradient.ElixirExpr.pp_expr/1
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:490: Gradient.ElixirExpr.pp_cons/1
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:68: Gradient.ElixirExpr.pp_expr/1
    (elixir 1.14.2) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2
    (elixir 1.14.2) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:115: Gradient.ElixirExpr.pp_expr/1
    (gradient 0.1.0) lib/gradient/elixir_expr.ex:17: Gradient.ElixirExpr.pp_expr_format/2
    (gradient 0.1.0) lib/gradient/elixir_fmt.ex:265: anonymous fn/4 in Gradient.ElixirFmt.pp_expr_fun/1
    (gradient 0.1.0) lib/gradient/elixir_fmt.ex:233: Gradient.ElixirFmt.format_expr_type_error/4
    (gradient 0.1.0) lib/gradient/elixir_fmt.ex:58: Gradient.ElixirFmt.print_error/2
    (gradient 0.1.0) lib/gradient/elixir_fmt.ex:44: anonymous fn/3 in Gradient.ElixirFmt.print_errors/2
    (elixir 1.14.2) lib/enum.ex:2468: Enum."-reduce/3-lists^foldl/2-0-"/3
    (gradient 0.1.0) lib/gradient/elixir_fmt.ex:42: Gradient.ElixirFmt.print_errors/2
    (gradient 0.1.0) lib/gradient.ex:88: Gradient.handle_elixir_ast/2
    (elixir 1.14.2) lib/enum.ex:1658: Enum."-map/2-lists^map/1-0-"/2

[erszcz]

Hey, @lukaszsamson!

Thanks for creating the ticket. It would help a lot if you could also attach the code the crash happens on.

[lukaszsamson]

I cannot share the repo but I'll try to isolate it

[erszcz]

I've been trying to reproduce this error, but after trying a number of different combinations I still don't know what generates the guards we see in the error message. These guards check for an argument being an atom, but neither nil, nor true, nor false:

[
  [
    {:call, [generated: true, location: 21],
     {:remote, [generated: true, location: 21],
      {:atom, [generated: true, location: 21], :erlang},
      {:atom, [generated: true, location: 21], :is_atom}},
     [{:var, [generated: true, location: 21], :_@1}]},
    {:op, [generated: true, location: 21], :"=/=",
     {:var, [generated: true, location: 21], :_@1},
     {:atom, [generated: true, location: 21], nil}},
    {:op, [generated: true, location: 21], :"=/=",
     {:var, [generated: true, location: 21], :_@1},
     {:atom, [generated: true, location: 21], true}},
    {:op, [generated: true, location: 21], :"=/=",
     {:var, [generated: true, location: 21], :_@1},
     {:atom, [generated: true, location: 21], false}}
  ]
]

In general, when defining guards in Elixir we have to use the and / or conjunctives. These are equivalent to andalso / orelse in Erlang, and these in turn always lead to the following form of guards in the abstract syntax tree:

iex(1)> :merl.quote('fun (X) when X > 1 andalso X < 5 -> ok end')
{:fun, 1,
 {:clauses,
  [
    {:clause, 1, [{:var, 1, :X}],
     [
       [
         {:op, 1, :andalso, {:op, 1, :>, {:var, 1, :X}, {:integer, 1, 1}},
          {:op, 1, :<, {:var, 1, :X}, {:integer, 1, 5}}}
       ]
     ], [{:atom, 1, :ok}]}
  ]}}

We can see there's a single op expression within a nested list, not a list of consecutive guard expressions as in the original error message:

       [
         {:op, 1, :andalso, {:op, 1, :>, {:var, 1, :X}, {:integer, 1, 1}},
          {:op, 1, :<, {:var, 1, :X}, {:integer, 1, 5}}}
       ]

In Erlang it's possible to generate a list of guards by using the , or ; conjunctives, too:

iex(2)> :merl.quote('fun (X) when X > 1, X < 5 -> ok end')
{:fun, 1,
 {:clauses,
  [
    {:clause, 1, [{:var, 1, :X}],
     [
       [
         {:op, 1, :>, {:var, 1, :X}, {:integer, 1, 1}},
         {:op, 1, :<, {:var, 1, :X}, {:integer, 1, 5}}
       ]
     ], [{:atom, 1, :ok}]}
  ]}}

This, in fact, generates a list of guards:

       [
         {:op, 1, :>, {:var, 1, :X}, {:integer, 1, 1}},
         {:op, 1, :<, {:var, 1, :X}, {:integer, 1, 5}}
       ]

However, as far as I know, Elixir doesn't have an equivalent of , / ; conjunctives for use in guard expressions. Moreover, [generated: true, location: 21] tells us that these guards were generated by the Elixir compiler, likely by some special form, on line 21 of some unknown file.

Without a hint about what code leads to this error it's going to be quite hard to debug further :|