How to use hcl write to set expressions with ${}?

I am trying to use hclwrite to generate .tf files.

According to the example in hclwrite Example, I can generate variables like foo = env.PATH, but I don't know how to generate more forms of expressions. For example, the following.

stage = "prod"
foo   = "hello${var.stage}"

when i set foo with

SetAttributeValue("foo", cty.StringVal("hello${bar}"))

i get

foo = "hello$${bar}"

The hclwrite tool currently has no facility to automatically generate arbitrary expressions. Its helper functions are limited only to generating plain references and literal values. SetAttributeValue is the one for literal values, and so that's why the library correctly escaped the ${ sequence in your string, to ensure that it will be interpreted literally.

If you want to construct a more elaborate expression then you'll need to do so manually by assembling the tokens that form the expression and then calling SetAttributeRaw instead.

In the case of your example, it looks like you'd need to generate the following eight tokens:

• TokenOQuote with the bytes "
• TokenQuotedLit with the bytes hello
• TokenTemplateInterp with the bytes ${
• TokenIdent with the bytes var
• TokenDot with the bytes .
• TokenIdent with the bytes stage
• TokenTemplateSeqEnd with the bytes }
• TokenCQuote with the bytes "

The SetAttributeValue function is automating the simpler case of generating three tokens: TokenOQuote, TokenQuotedLit, TokenCQuote.

You can potentially automate the creation of tokens for the var.stage portion of this expression by using TokensForTraversal, which is what SetAttributeTraversal does internally. However, unless you already have a parsed hcl.Traversal representing var.stage, or unless you need things to be more dynamic than you've shown in practice, I expect that it would take more code to construct that input traversal than to just write out the three tokens literally as I showed above.