The range the probe reaches, or an Issue stating why it reaches nothing: "unknown property path"
where the path names a member no alternative carries or steps past one that cannot be stepped past,
"multiple aggregate transforms" where the pipe combines values more than once, or
"incompatible transform input" where no alternative survives the transforms
TraceError Where a deferred definition reaches itself, leaving the shape it states undefined, or where it states a target that doesn't narrow the one the link it is reached through overrides, or where a union the path crosses declares a shared member name inconsistently across its branches
TypeError Where probe is not a well-formed probe
Resolves the values a probe reaches.
Yields the range a probe resolves to against a shape: the shapes its values may be drawn from and how many of them it reaches, so that a caller may type a projection binding or a constraint operand without walking the shape itself. A path stepping through several alternatives reaches each in turn, and the range it yields envelopes them all; discrimination to the single branch a value belongs to is settled separately, against the value.
What a path reaches
A path steps across the members of the resources it reaches, crossing a link to the resource it points at and entering each alternative of a union in turn; an alternative lacking the next member is dropped, and the path fails only where none carries it. The members naming and typing a resource are reached as a scalar IRI, so a path may end on one but never step past it. A localised member is likewise terminal.
A path taking no step reaches the shape it starts from, as it stands: a link stays a link, reached as the identifier naming its target unless a template expands it, and is crossed only by a step that actually names a member.
What a pipe yields
A pipe applies its transforms to whatever the path reached, dropping an alternative the transforms cannot act on: within a union this keeps the compatible alternatives alone, so a pipe stands as long as one survives. A localised value is read through the content language negotiation settles on, as ordinary text. A transform stating it yields the same type reproduces the input shape verbatim where it keeps values within the domain they were drawn from, and widens to the bare numeric type where it combines them and leaves that domain behind.
How many values it reaches
Bounds accumulate multiplicatively along a path and envelope across alternatives: the lowest lower bound and the highest upper bound win, and an unstated bound absorbs, leaving that end unbounded. A pipe that always yields a value floors the count at one, an aggregate caps it at one, and any other transform leaves the count unstated.