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Language and Compatibility Settings

Configuration blocks of type language can be used to explicitly declare which versions of OpenTofu a module is intended to be compatible with, and what edition of the OpenTofu language the module is written for.

OpenTofu also supports some other settings as an aid to writing modules that are cross-compatible between OpenTofu and Terraform, described later in this page.

language Block Syntax​

Language-related settings are written in language blocks:

Code Block
language {
  # ...
}

Blocks of this type are accepted only in OpenTofu v1.12 and later. If you are writing a module that must be compatible with older versions of OpenTofu, refer to Compatibility with Older Versions and Other Software below for an alternative to specifying which OpenTofu versions your module is intended to be compatible with.

Declaring Implementation Compatibility​

A compatible_with block nested inside language specifies which software a module is intended to be compatible with:

Code Block
language {
  compatible_with {
    opentofu = ">= 1.12"
  }
}

OpenTofu ignores everything except an opentofu argument inside a compatible_with block. Other software which interacts with OpenTofu modules may define its own argument name for describing additional compatibility constraints with that software, in which case those arguments should be mentioned in the documentation for that software.

Language Editions and Experimental Features​

The language block type supports two additional arguments that are not useful in current OpenTofu versions, but may be extended in future versions of OpenTofu:

  • edition: Specifies a "language edition" that the module is written for, which in a future version of OpenTofu might opt in to one or more language features that are not completely backward compatible with previous versions of OpenTofu.

    Modules written for today's OpenTofu should typically not set this argument at all. It's valid to assign the keyword tofu2024 to represent the current edition of the OpenTofu language, but that has no useful effect because that edition will always be the default.

  • experiments: Specifies a set of experimental language features the module is using.

    The current version of OpenTofu has no active experiments, so the only valid expression to assign to this argument is the empty set [], which is also the default. Modules written for today's OpenTofu should typically not set this argument at all.

Current OpenTofu only has basic handling of these arguments enough to return helpful error messages if different possibilities are introduced in future versions of OpenTofu.

Compatibility with Older Versions and Other Software​

The language block type is not supported in OpenTofu v1.11 or earlier, and also not recognized by any version of Terraform that's available at the time of writing this documentation.

To allow writing modules that declare compatibility with older versions of OpenTofu or with Terraform, OpenTofu supports an alternative way to specify version constraints, which uses a top-level block type named terraform only for compatibility with Terraform.

Code Block
terraform {
  required_version = ">= 0.11"
}

Older versions of OpenTofu interpreted this setting as specifying an OpenTofu version constraint in all cases, which is bothersome when using a module that was originally written for Terraform and so uses this to specify a Terraform version constraint.

To write a cross-compatible module that works with older versions of OpenTofu and with Terraform, we recommend the following strategy:

  • Create a versions.tofu file which contains either a required_version argument (if older version compatibility is important) or a language block specifying which versions of OpenTofu the module is intended to be compatible with.
  • Create a versions.tf file which contains a required_version argument specifying which versions of Terraform the module is intended to be compatible with.

When both versions.tofu and versions.tf files are present, OpenTofu will ignore the versions.tf file assuming that it is intended for Terraform's use. At the time of writing this documentation Terraform does not read .tofu files, and so Terraform should similarly ignore versions.tofu.

Other uses of terraform blocks​

This page is focused on specifying OpenTofu version compatibility and language edition settings, but a terraform block accepts a number of other unrelated settings described elsewhere: