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Return errors as strings, not resources
This change proposes a different approach to simplifying errors than PR WebAssembly#75 (errors as records). Here we take that approach one step further and completely remove the error code portion entirely, returning only a `string` in the error case. Is this an anti-pattern, though? Some would argue that this is the case, since users might have to resort to string-matching for custom error handling. Others, however, might argue that the error codes were low value anyways since we expect to only see one or maybe two variants as failures to a single call. In any case, this PR offers an alternative to WebAssembly#75 for discussion sake.
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‎ml.md‎

Lines changed: 10 additions & 95 deletions
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@@ -10,7 +10,6 @@ Then, the user passes <em>tensor</em> inputs to the <em>graph</em>, computes the
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<li>Imports:
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<ul>
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<li>interface <a href="#wasi_nn_tensor_0_2_0_rc_2024_06_25"><code>wasi:nn/[email protected]</code></a></li>
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<li>interface <a href="#wasi_nn_errors_0_2_0_rc_2024_06_25"><code>wasi:nn/[email protected]</code></a></li>
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<li>interface <a href="#wasi_nn_inference_0_2_0_rc_2024_06_25"><code>wasi:nn/[email protected]</code></a></li>
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<li>interface <a href="#wasi_nn_graph_0_2_0_rc_2024_06_25"><code>wasi:nn/[email protected]</code></a></li>
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</ul>
@@ -90,94 +89,13 @@ containing a single value, use <code>[1]</code> for the tensor dimensions.</p>
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<ul>
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<li><a name="method_tensor_data.0"></a> <a href="#tensor_data"><a href="#tensor_data"><code>tensor-data</code></a></a></li>
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</ul>
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<h2><a name="wasi_nn_errors_0_2_0_rc_2024_06_25"></a>Import interface wasi:nn/[email protected]</h2>
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<p>TODO: create function-specific errors (https://github.com/WebAssembly/wasi-nn/issues/42)</p>
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<hr />
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<h3>Types</h3>
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<h4><a name="error_code"></a><code>enum error-code</code></h4>
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<h5>Enum Cases</h5>
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<ul>
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<li>
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<p><a name="error_code.invalid_argument"></a><code>invalid-argument</code></p>
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<p>Caller module passed an invalid argument.
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</li>
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<li>
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<p><a name="error_code.invalid_encoding"></a><code>invalid-encoding</code></p>
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<p>Invalid encoding.
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</li>
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<li>
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<p><a name="error_code.timeout"></a><code>timeout</code></p>
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<p>The operation timed out.
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</li>
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<li>
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<p><a name="error_code.runtime_error"></a><code>runtime-error</code></p>
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<p>Runtime Error.
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</li>
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<li>
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<p><a name="error_code.unsupported_operation"></a><code>unsupported-operation</code></p>
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<p>Unsupported operation.
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</li>
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<li>
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<p><a name="error_code.too_large"></a><code>too-large</code></p>
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<p>Graph is too large.
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</li>
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<li>
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<p><a name="error_code.not_found"></a><code>not-found</code></p>
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<p>Graph not found.
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</li>
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<li>
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<p><a name="error_code.security"></a><code>security</code></p>
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<p>The operation is insecure or has insufficient privilege to be performed.
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e.g., cannot access a hardware feature requested
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</li>
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<li>
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<p><a name="error_code.unknown"></a><code>unknown</code></p>
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<p>The operation failed for an unspecified reason.
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</li>
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</ul>
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<h4><a name="error"></a><code>resource error</code></h4>
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<hr />
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<h3>Functions</h3>
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<h4><a name="constructor_error"></a><code>[constructor]error: func</code></h4>
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<h5>Params</h5>
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<ul>
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<li><a name="constructor_error.code"></a><code>code</code>: <a href="#error_code"><a href="#error_code"><code>error-code</code></a></a></li>
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<li><a name="constructor_error.data"></a><code>data</code>: <code>string</code></li>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="constructor_error.0"></a> own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;</li>
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</ul>
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<h4><a name="method_error_code"></a><code>[method]error.code: func</code></h4>
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<p>Return the error code.</p>
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<h5>Params</h5>
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<ul>
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<li><a name="method_error_code.self"></a><code>self</code>: borrow&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;</li>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_error_code.0"></a> <a href="#error_code"><a href="#error_code"><code>error-code</code></a></a></li>
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</ul>
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<h4><a name="method_error_data"></a><code>[method]error.data: func</code></h4>
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<p>Errors can propagated with backend specific status through a string value.</p>
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<h5>Params</h5>
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<ul>
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<li><a name="method_error_data.self"></a><code>self</code>: borrow&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;</li>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_error_data.0"></a> <code>string</code></li>
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</ul>
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<h2><a name="wasi_nn_inference_0_2_0_rc_2024_06_25"></a>Import interface wasi:nn/[email protected]</h2>
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<p>An inference &quot;session&quot; is encapsulated by a <a href="#graph_execution_context"><code>graph-execution-context</code></a>. This structure binds a
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<a href="#graph"><code>graph</code></a> to input tensors before <code>compute</code>-ing an inference:</p>
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<hr />
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<h3>Types</h3>
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<h4><a name="error"></a><code>type error</code></h4>
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<p><a href="#error"><a href="#error"><code>error</code></a></a></p>
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<p>
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#### <a name="tensor"></a>`type tensor`
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[`tensor`](#tensor)
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<h4><a name="tensor"></a><code>type tensor</code></h4>
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<p><a href="#tensor"><a href="#tensor"><code>tensor</code></a></a></p>
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<p>
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#### <a name="tensor_data"></a>`type tensor-data`
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[`tensor-data`](#tensor_data)
@@ -197,7 +115,7 @@ e.g., cannot access a hardware feature requested
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_graph_execution_context_set_input.0"></a> result&lt;_, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="method_graph_execution_context_set_input.0"></a> result&lt;_, <code>string</code>&gt;</li>
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</ul>
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<h4><a name="method_graph_execution_context_compute"></a><code>[method]graph-execution-context.compute: func</code></h4>
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<p>Compute the inference on the given inputs.</p>
@@ -210,7 +128,7 @@ https://github.com/WebAssembly/wasi-nn/issues/43.</p>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_graph_execution_context_compute.0"></a> result&lt;_, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="method_graph_execution_context_compute.0"></a> result&lt;_, <code>string</code>&gt;</li>
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</ul>
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<h4><a name="method_graph_execution_context_get_output"></a><code>[method]graph-execution-context.get-output: func</code></h4>
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<p>Extract the outputs after inference.</p>
@@ -221,18 +139,15 @@ https://github.com/WebAssembly/wasi-nn/issues/43.</p>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_graph_execution_context_get_output.0"></a> result&lt;own&lt;<a href="#tensor"><a href="#tensor"><code>tensor</code></a></a>&gt;, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="method_graph_execution_context_get_output.0"></a> result&lt;own&lt;<a href="#tensor"><a href="#tensor"><code>tensor</code></a></a>&gt;, <code>string</code>&gt;</li>
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</ul>
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<h2><a name="wasi_nn_graph_0_2_0_rc_2024_06_25"></a>Import interface wasi:nn/[email protected]</h2>
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<p>A <a href="#graph"><code>graph</code></a> is a loaded instance of a specific ML model (e.g., MobileNet) for a specific ML
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framework (e.g., TensorFlow):</p>
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<hr />
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<h3>Types</h3>
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<h4><a name="error"></a><code>type error</code></h4>
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<p><a href="#error"><a href="#error"><code>error</code></a></a></p>
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<p>
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#### <a name="tensor"></a>`type tensor`
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[`tensor`](#tensor)
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<h4><a name="tensor"></a><code>type tensor</code></h4>
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<p><a href="#tensor"><a href="#tensor"><code>tensor</code></a></a></p>
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<p>
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#### <a name="graph_execution_context"></a>`type graph-execution-context`
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[`graph-execution-context`](#graph_execution_context)
@@ -274,7 +189,7 @@ graph IR in parts (e.g., OpenVINO stores its IR and weights separately).</p>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="method_graph_init_execution_context.0"></a> result&lt;own&lt;<a href="#graph_execution_context"><a href="#graph_execution_context"><code>graph-execution-context</code></a></a>&gt;, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="method_graph_init_execution_context.0"></a> result&lt;own&lt;<a href="#graph_execution_context"><a href="#graph_execution_context"><code>graph-execution-context</code></a></a>&gt;, <code>string</code>&gt;</li>
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</ul>
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<h4><a name="load"></a><code>load: func</code></h4>
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<p>Load a <a href="#graph"><code>graph</code></a> from an opaque sequence of bytes to use for inference.</p>
@@ -286,7 +201,7 @@ graph IR in parts (e.g., OpenVINO stores its IR and weights separately).</p>
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="load.0"></a> result&lt;own&lt;<a href="#graph"><a href="#graph"><code>graph</code></a></a>&gt;, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="load.0"></a> result&lt;own&lt;<a href="#graph"><a href="#graph"><code>graph</code></a></a>&gt;, <code>string</code>&gt;</li>
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</ul>
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<h4><a name="load_by_name"></a><code>load-by-name: func</code></h4>
292207
<p>Load a <a href="#graph"><code>graph</code></a> by name.</p>
@@ -299,5 +214,5 @@ range from simple to complex (e.g., URLs?) and caching mechanisms of various kin
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</ul>
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<h5>Return values</h5>
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<ul>
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<li><a name="load_by_name.0"></a> result&lt;own&lt;<a href="#graph"><a href="#graph"><code>graph</code></a></a>&gt;, own&lt;<a href="#error"><a href="#error"><code>error</code></a></a>&gt;&gt;</li>
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<li><a name="load_by_name.0"></a> result&lt;own&lt;<a href="#graph"><a href="#graph"><code>graph</code></a></a>&gt;, <code>string</code>&gt;</li>
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</ul>

‎wit/wasi-nn.wit‎

Lines changed: 6 additions & 44 deletions
Original file line numberDiff line numberDiff line change
@@ -12,7 +12,6 @@ world ml {
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import tensor;
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import graph;
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import inference;
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import errors;
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}
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/// All inputs and outputs to an ML inference are represented as `tensor`s.
@@ -61,13 +60,12 @@ interface tensor {
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/// A `graph` is a loaded instance of a specific ML model (e.g., MobileNet) for a specific ML
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/// framework (e.g., TensorFlow):
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interface graph {
64-
use errors.{error};
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use tensor.{tensor};
6664
use inference.{graph-execution-context};
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6866
/// An execution graph for performing inference (i.e., a model).
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resource graph {
70-
init-execution-context: func() -> result<graph-execution-context, error>;
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init-execution-context: func() -> result<graph-execution-context, string>;
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}
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/// Describes the encoding of the graph. This allows the API to be implemented by various
@@ -96,20 +94,19 @@ interface graph {
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type graph-builder = list<u8>;
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/// Load a `graph` from an opaque sequence of bytes to use for inference.
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load: func(builder: list<graph-builder>, encoding: graph-encoding, target: execution-target) -> result<graph, error>;
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load: func(builder: list<graph-builder>, encoding: graph-encoding, target: execution-target) -> result<graph, string>;
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/// Load a `graph` by name.
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///
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/// How the host expects the names to be passed and how it stores the graphs for retrieval via
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/// this function is **implementation-specific**. This allows hosts to choose name schemes that
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/// range from simple to complex (e.g., URLs?) and caching mechanisms of various kinds.
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load-by-name: func(name: string) -> result<graph, error>;
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load-by-name: func(name: string) -> result<graph, string>;
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}
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/// An inference "session" is encapsulated by a `graph-execution-context`. This structure binds a
110108
/// `graph` to input tensors before `compute`-ing an inference:
111109
interface inference {
112-
use errors.{error};
113110
use tensor.{tensor, tensor-data};
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115112
/// Bind a `graph` to the input and output tensors for an inference.
@@ -118,51 +115,16 @@ interface inference {
118115
/// (https://github.com/WebAssembly/wasi-nn/issues/43)
119116
resource graph-execution-context {
120117
/// Define the inputs to use for inference.
121-
set-input: func(name: string, tensor: tensor) -> result<_, error>;
118+
set-input: func(name: string, tensor: tensor) -> result<_, string>;
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123120
/// Compute the inference on the given inputs.
124121
///
125122
/// Note the expected sequence of calls: `set-input`, `compute`, `get-output`. TODO: this
126123
/// expectation could be removed as a part of
127124
/// https://github.com/WebAssembly/wasi-nn/issues/43.
128-
compute: func() -> result<_, error>;
125+
compute: func() -> result<_, string>;
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130127
/// Extract the outputs after inference.
131-
get-output: func(name: string) -> result<tensor, error>;
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}
133-
}
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/// TODO: create function-specific errors (https://github.com/WebAssembly/wasi-nn/issues/42)
136-
interface errors {
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enum error-code {
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// Caller module passed an invalid argument.
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invalid-argument,
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// Invalid encoding.
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invalid-encoding,
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// The operation timed out.
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timeout,
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// Runtime Error.
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runtime-error,
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// Unsupported operation.
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unsupported-operation,
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// Graph is too large.
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too-large,
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// Graph not found.
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not-found,
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// The operation is insecure or has insufficient privilege to be performed.
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// e.g., cannot access a hardware feature requested
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security,
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// The operation failed for an unspecified reason.
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unknown
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}
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resource error {
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constructor(code: error-code, data: string);
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/// Return the error code.
163-
code: func() -> error-code;
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/// Errors can propagated with backend specific status through a string value.
166-
data: func() -> string;
128+
get-output: func(name: string) -> result<tensor, string>;
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}
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}

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