Add Cabal-7070

Author: philderbeast

message-index/messages/Cabal-7070/index.md

@@ -0,0 +1,81 @@
+---
+title: Run whole executable
+summary: Cabal runs executables but not modules
+severity: error
+introduced: 3.12.1.0
+---
+
+Self-contained, single-file Cabal scripts cannot also be part of a package, as
+an executable or listed as a module of any package component.
+
+Let's say we have a Cabal script, `myscript.hs`:
+
+```haskell
+#!/usr/bin/env cabal
+{- cabal:
+build-depends:
+    base,
+    haskell-say ^>=1.0.0.0
+-}
+
+import HaskellSay (haskellSay)
+
+main :: IO ()
+main = haskellSay "Hello, Haskell!"
+```
+
+We can run this with `cabal run` or with `./myscript.hs`.
+
+```
+$ cabal run myscript.hs
+    ________________________________________________________
+   /                                                        \
+  | Hello, Haskell!                                          |
+   \____       _____________________________________________/
+        \    /
+         \  /
+          \/
+    _____   _____
+    \    \  \    \
+     \    \  \    \
+      \    \  \    \
+       \    \  \    \  \-----------|
+        \    \  \    \  \          |
+         \    \  \    \  \---------|
+         /    /  /     \
+        /    /  /       \  \-------|
+       /    /  /    ^    \  \      |
+      /    /  /    / \    \  \ ----|
+     /    /  /    /   \    \
+    /____/  /____/     \____\
+```
+
+Once we add this script to a package then we lose the ability to run the script
+by specifying its module name. An executable name from the package must be used
+instead. Moreover, any module included in a package is not runnable by itself
+with `cabal run`.
+
+```
+$ cat script-exclusivity.cabal
+cabal-version: 3.0
+name: script-exclusitivity
+version: 1
+
+executable my-script-exe
+    build-depends:
+        base,
+        haskell-say
+    main-is: myscript.hs
+
+$ ./myscript.hs
+Error: [Cabal-7070]
+The run command can only run an executable as a whole, not files or modules
+within them, but the target 'myscript.hs' refers to the file myscript.hs in the
+executable my-script-exe.
+
+$ cabal run myscript.hs
+Error: [Cabal-7070]
+The run command can only run an executable as a whole, not files or modules
+within them, but the target 'myscript.hs' refers to the file myscript.hs in the
+executable my-script-exe.
+```