Rendering Values in a Step Function: extract YYYY-MM from 2022-10-20 - amazon-web-services

Question

I am trying to get a date prefix from a date in an AWS Step Function. When I try to call a variable from a previous step, that variable doesn't render. Here is the step function code:
{
"Comment": "Set and use a new variable in Step Functions",
"StartAt": "Set Date Prefix",
"States": {
"Set Date Prefix": {
"Type": "Pass",
"Result": {
"date_prefix": "${$$.Execution.Input.date.substr(0,7)}"
},
"ResultPath": "$.date_prefix",
"Next": "Use Date Prefix"
},
"Use Date Prefix": {
"Type": "Pass",
"Result": {
"date_prefix_used": "$.date_prefix"
},
"End": true
}
}
}
When I pass the following input:
{
"date": "2022-10-20"
}
I get the following as output:
{
"date_prefix_used": "$.date_prefix"
}
when I should have gotten:
{
"date_prefix": "2022-10",
"date_prefix_used": "2022-10"
}
What am I doing wrong?

Answer 1

You can intinsically extract the YYYY-MM from a date string input like 2022-10-20 with two Pass states. A Pass state can apply substitutions and State Machine intrinsic functions, but in the Parameters field, not the Result field. Remember the .$ suffix on keys whose values have substitutions. Otherwise, the values will be treated as literals.
{
"Comment": "Set and use a new variable in Step Functions",
"StartAt": "SplitDate",
"States": {
"SplitDate": {
"Type": "Pass",
"Parameters": {
"date_components.$": "States.StringSplit($.date, '-')"
},
"Next": "GetPrefix"
},
"GetPrefix": {
"Type": "Pass",
"InputPath": "$.date_components",
"Parameters": {
"date_prefix.$": "States.Format('{}-{}',States.ArrayGetItem($, 0), States.ArrayGetItem($, 1))"
},
"End": true
}
}
}
The Split Date task splits the date with States.StringSplit.
{
"date_components": [ "2022", "10", "20" ]
}
The GetPrefix task reassembles the the year and month with States.Format and States.ArrayGetItem:
{
"date_prefix": "2022-10"
}