Argo workflow when a condition for substring match - kubernetes

Question

I want to execute a task in Argo workflow if a string starts with a particular substring.
For example, my string is tests/dev-or.yaml and I want to execute task if my string starts with tasks/
Here is my workflow but the condition is not being validated properly
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: conditional-
spec:
entrypoint: conditional-example
arguments:
parameters:
- name: should-print
value: "tests/dev-or.yaml"
templates:
- name: conditional-example
inputs:
parameters:
- name: should-print
steps:
- - name: print-hello
template: whalesay
when: "{{inputs.parameters.should-print }} startsWith 'tests/'"
- name: whalesay
container:
image: docker/whalesay:latest
command: [sh, -c]
args: ["cowsay hello"]
Below is the error it is giving when I run the workflow
WorkflowFailed 7s workflow-controller Invalid 'when' expression 'tests/dev-or.yaml startsWith 'tests/'': Unable to access unexported field 'yaml' in token 'or.yaml'
Seems it is not accepting -, .yaml and / while evaluating the when condition.
Any mistake am making in my workflow? What's the right way to use this condition?

Answer 1

tl;dr - use this: when: "'{{inputs.parameters.should-print}}' =~ '^tests/'"
Parameter substitution happens before the when expression is evaluated. So the when expression is actually tests/dev-or.yaml startsWith 'tests/'. As you can see, the first string needs quotation marks.
But even if you had when: "'{{inputs.parameters.should-print}}' startsWith 'tests/'" (single quotes added), the expression would fail with this error: Cannot transition token types from STRING [tests/dev-or.yaml] to VARIABLE [startsWith].
Argo Workflows conditionals are evaluated as govaluate expressions. govaluate does not have any built-in functions, and Argo Workflows does not augment it with any functions. So startsWith is not defined.
Instead, you should use govaluate's regex comparator. The expression will look like this: when: "'{{inputs.parameters.should-print}}' =~ '^tests/'".
This is the functional Workflow:
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: conditional-
spec:
entrypoint: conditional-example
arguments:
parameters:
- name: should-print
value: "tests/dev-or.yaml"
templates:
- name: conditional-example
inputs:
parameters:
- name: should-print
steps:
- - name: print-hello
template: whalesay
when: "'{{inputs.parameters.should-print}}' =~ '^tests/'"
- name: whalesay
container:
image: docker/whalesay:latest
command: [sh, -c]
args: ["cowsay hello"]