On 30 October 2016 at 19:40, Waldek Hebisch <[email protected]> wrote:
>
> Before using a function Spad runtime first has to compute
> types appearing in function signature. So before call to 'ap'
> we have to compute 'A(S->S)' with 'A' replaced by 'AMaybe'.
> The problem is that constructors (in particular 'AMaybe')
> use different calling convention than normal functions.
Notice that 'A(S->S)' only appears in the signature of 'ap'. As far as
I understand it functions in signatures are never "called". In terms
of computation all that happens is that parameters such as 'A' and
'S'; are substituted for values.
> More precisely, normal functions have an extra hidden
> argument which normally is the domain implementing the
> function. Constructors have no such extra argument.
> You may think that extra argument can be safely ignored,
> but unfortunately Lisp will signal mismatch if number
> of declared arguments does not agree with number of
> actual arguments.
But Lisp evaluation is never called on 'A(S->S)'.
> Inside Applicative symbol 'A' has
> type 'Type->Type' so is considered to be an ordinary
> function so call will add the hidden argument.
>
it is not used in a context where it must be evaluated.
In fact the apparent function call is only "decoration" in any case.
The following variant of the code gives exactly the same result:
)abbrev category APPL Applicative
Applicative(S : Type, A: Type) : Category == Type with
pure: S -> A(S)
ap: (A(S->S),A(S)) -> A(S)
> To be clear: we _should_ use the same calling convention
> for functions and constructors. But the above is how
> current runtime/compiler works and required changes are
> rather large...
>
Perhaps the following example
--
)abbrev category APPL Applicative
Applicative(S : Type, A: (Type->Type)) : Category == Type with
pure: S -> A(S)
ap: (A(S->S),A(S)) -> A(S)
ff: () -> Integer
add
ff() == 0$Integer
--
manifests a problem similar to that to which you refer. This compiles
and works exactly as I indicated before but if I try to call the dummy
exported function 'ff' I get the following unexpected message:
(1) -> ff()$AMaybe(Integer)
Internal Error
Unexpected error in call to system function evalSlotDomain
So it seems that adding a default implementation to even an unrelated
function can create a problem.
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