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Forbid constant <op> constant comparisons for strings #229

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Dec 23, 2019
Merged

Forbid constant <op> constant comparisons for strings #229

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ebc9139
Forbid constant <op> constant comparisons for Strings, fixes #157
rartino Dec 20, 2019
4ce37be
Minor text fixes suggested in review
rartino Dec 21, 2019
7a3887e
Minor text adustment for clarity about types.
rartino Dec 21, 2019
3114ef1
Remove redundant also
rartino Dec 23, 2019
0fff3c3
Set aside formally outside specification as an indented block.
rartino Dec 23, 2019
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10 changes: 8 additions & 2 deletions optimade.rst
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Expand Up @@ -1258,16 +1258,22 @@ Numeric and String comparisons
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Comparisons involving Numeric and String properties can be expressed using the usual comparison operators: '<', '>', '<=', '>=', '=', '!='.
Implementations MUST support comparisons on the form::
Implementations MUST support comparisons in the forms::

identifier <operator> constant
constant <operator> identifier

Where 'identifier' is a property name and 'constant' is either a numerical or string type constant. However, implementations MAY OPTIONALLY support comparisons with identifiers also on both sides, and comparisons with values on both sides, i.e., on the forms::
Where :filter-fragment:`identifier` is a property name and :filter-fragment:`constant` is either a numerical or string type constant.

Implementations MAY also support comparisons with identifiers also on both sides, and comparisons with numerical type constants on both sides, i.e., in the forms::
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identifier <operator> identifier
constant <operator> constant

However, the latter form, :filter-fragment:`constant <operator> constant` where the constants are strings MUST return the error :http-error:`501 Not Implemented`.
The reason is that filter language strings can refer to data of different data types (e.g., strings and timestamps), and thus this construct is not unambigous.
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The OPTiMaDe specification will strive to address this issue in a future version.

Examples:

- :filter:`nelements > 3`
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