[Vm-dev] FFI Plugin | Proposal: Add #doesNotCoerce:for: (like #doesNotUnderstand:)

[Marcel Taeumel]

Hi all!

We have #doesNotUnderstand:, which is sent to the receiver when a message cannot be understood. Default reaction in Object is to raise MessageNotUnderstood exception.

What if we could have a similar mechanism for FFI calls: #doesNotCoerce:for:. The receiver would be the one that made that call, should be an instance of ExternalLibrary, but could be any object. The arguments would be a pair of (argObject, argType) and the message (like the argument for doesNotUnderstand:) that represents the FFI call.

The pair of (argObject, argType) could be expressed as FFICallArgument:

FFICallArgument
   argObject  ... <Object>
   argType ... <ExternalType>

I suppose that such a new class would have to be reserved in the special objects array. An alternative would be to just use Array. Keep it simple. ;-)

Default reaction could be to implement #doesNotCoerce:for: in Object (as *FFI-Kernel extension) to raise FFICallArgumentNotCoerced exception (to be implemented). Also, we could implement manual coercion in ExternaLibrary to, for example, wrap atomics into their aliasing structures.

ExternalLibrary >> #doesNotCoerce: callArg for: message
   | argObject argType argIndex wrappedArg |
   argObject := callArg first. "or argObject accessor"
   argType := callArg second. "or argType accessor"
   argIndex := message arguments indexOf: argObject.
   wrappedArg := argType referentClass fromHandle: argObject.
   message arguments beWritableObject. "Just in case ;-)"
   message at: argIndex put: wrappedArg.
   ^ message sendTo: self

Please share your thoughts. :-)

Best,
Marcel
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