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Add warning and note about cabal init and script #10327

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Add warning and note about cabal init and script #10327

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35 changes: 34 additions & 1 deletion doc/getting-started.rst
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Expand Up @@ -58,6 +58,18 @@ The ``myapp.cabal`` file is a package description file, commonly referred to as
hs-source-dirs: app
default-language: Haskell2010

.. warning::

The version bounds on base, a boot library distributed with GHC
[#boot-packages]_, are tied to the GHC version visible when ``cabal init``
is run. If run with a later version of GHC you might see a difference in the
version bounds.

.. code-block:: diff

- build-depends: base ^>=4.19.0.0
+ build-depends: base ^>=4.20.0.0

It contains metadata (package name and version, author name, license, etc.) and sections
to define package components. Components can be used to split large codebases into smaller,
more managable building blocks.
Expand Down Expand Up @@ -189,7 +201,7 @@ the following file named ``myscript``:
#!/usr/bin/env cabal
{- cabal:
build-depends:
base ^>=4.19.0.0,
base,
haskell-say ^>=1.0.0.0
-}

Expand All @@ -198,6 +210,21 @@ the following file named ``myscript``:
main :: IO ()
main = haskellSay "Hello, Haskell!"

.. note::

Widening or dropping version bound constraints on *packages included with
the compiler* [#boot-packages]_, like ``base``, may allow single-file
scripts to run with a wider range of compiler versions.

.. code-block:: diff

build-depends:
- base ^>=4.19.0.0,
+ base,

The necessary sections of a ``.cabal`` file are placed
directly into the script as a comment.

The necessary sections of a package description that would otherwise be in a
``.cabal`` file are placed directly into the script as a comment.

Expand Down Expand Up @@ -237,3 +264,9 @@ some of the resources on the Haskell website's `documentation page
Cabal on the :doc:`What Cabal does <cabal-context>` page.

.. _Cabal-7070: https://errors.haskell.org/messages/Cabal-7070/

.. [#boot-packages] Packages included with the compiler are also called boot
packages. Each GHC compiler version has accompanying `release notes`_ that
list these included packages.

.. _release notes: https://downloads.haskell.org/ghc/latest/docs/users_guide/release-notes.html
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