specify YAML version, which schema

[mr-c]

the CWL reference runner supports YAML 1.2 with the core schema via ruamel.yaml

Java based CWL parsers tend to either use snakeyaml which is YAML 1.1 only. snakeyaml-engine supports YAML 1.2, but only the JSON schema, and not the core schema.

Some fun examples of why this matters:

1. False (capital F) being interpreted as a boolean as in

   cwl-v1.2/tests/stderr.cwl

   Line 13 in b35840e

   - { valueFrom: "echo foo 1>&2", shellQuote: False }

   is not supported in YAML 1.2 unless the parser supports the core schema, not just the JSON schema.
2. Yes (unquoted) is interpreted as a boolean in YAML 1.1, but not YAML 1.2

[mr-c]

Here's a way of using snakeyaml-engine to approximate YAML 1.2 with the Core + JSON schema: https://github.com/common-workflow-language/schema_salad/pull/375/files#diff-1fe41c7baff746ee2b3d83564fbe12aa60b05cce03e43e86ffd6d1b8bac6d9aa

[tetron]

So the schema salad spec says to use the YAML 1.2 "JSON schema" and not the Core schema:

https://www.commonwl.org/v1.2/SchemaSalad.html#Syntax

It sounds like we need to fix some conformance tests. I would support adding additional conformance tests that assert behavior in other corner cases. I know the existence of alternate symbols for boolean values is a common complaint by other projects using YAML.