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LDC-E2108 — Operands of incompatible kinds

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# LDC-E2108 — Operands of incompatible kinds

## What it means

Arithmetic and comparison work on matching kinds: adding a date to
money, or ordering values that have no order, is rejected where it
stands. The language performs no silent conversions between kinds —
each operand must already be what the operation expects.

Rewriting the expression so both sides share a kind, or picking the
operation that fits the kinds at hand, fixes it. The message names the
two kinds found, which points straight at the side to change.

## Example

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

relation out(x: Text, a: Money) kind institutional;
relation m(x: Text, a: Money) kind empirical;
relation d(x: Text, t: Date) kind empirical;
rule R strict { for x: Text; for a: Money; for t: Date; when m(x, a) and d(x, t); then out(x, (t + a)); }
```

## Fix

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

relation out(x: Text, a: Money) kind institutional;
relation m(x: Text, a: Money) kind empirical;
relation d(x: Text, t: Date) kind empirical;
rule R strict { for x: Text; for a: Money; for b: Money; when m(x, a) and m(x, b); then out(x, (a + b)); }
```

## Engine message

The engine reports this in its own wording:

```text
example.law:8:96: error LDC-E2108: «+»: incompatible kinds Date and Money — §58 requires matching kinds for addition and subtraction; the §49 scalar rule applies ONLY to multiplication (§48: no implicit conversions)
```

## Related

- [Expressions and quantities](/constructs/expressions-quantities/) — which operations fit which kinds.