Proposal for expansion of expression language

art-decisions/proposals/2023-03-14-expression-expansion.md

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+# Expansion of Workflow Expression Language
+
+## Background
+
+The current state of the expression language allows users to retrieve values from inputs and step output. That is enough for usable workloads, but is insuffient for many use cases.
+
+Examples of situations which are limited right now:
+- Can't import values from files (will be done with a function)
+- Can't conditionally run sections (will be done with a conditional expression and a sub-workflow)
+- Can't do comparisons or math with known values for inputs and outputs. (Will be done with conditional expressions and mathematical expressions)
+
+This proposal will make it so less manual work is required to get a workflow working.
+
+## Proposal
+
+Proposed changes:
+- Make subexpressions default within map access since it is unintuitive as-is, removing the old () syntax. Moving from `map[(sub-expression)]` to `map[sub-expression]`
+- Add functions. Would be added as a root item, and therefore would work where subexpressions are supported. That would also make it so an expression can be exclusively a function call. The functions will be strongly typed, with a standardized interface for their inputs and outputs, allowing type verification.
+- Add binary comparison operators. This would need to be another root item. This would be simple. `expression ==|!=|<|>|<=|>= expression`. Types must match, or else there will be a type error.
+- Mathematical expressions. These would be simple binary mathematical operators, with the potential for parhenthases for ordering changes. Will be strongly typed.
+
+Detailed issues for proposed changes:
+- [Functions](https://github.com/arcalot/arcaflow-expressions/issues/1)
+- [Binary Comparison Operators](https://github.com/arcalot/arcaflow-expressions/issues/2)
+- [Mathematical Expressions](https://github.com/arcalot/arcaflow-expressions/issues/3)
+
+### Priorities 
+- The function calls and comparison operators will be the top priorities
+- Mathematical expressions will be lower priority, with most of the work based on the work for the comparison operators.
+
+### Approval
+By approving this proposal, you are agree to the addition of functions, binary comparison operators, and mathematical expressions to the language. Not necessarily the way it's implemented.
+
+Discussion on how it is implemented can be debated in the issues on the expression repository linked above.
+
+## Drawbacks
+- It adds complexity to the language