unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent-multi-diff.t/A

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+I am file A.

unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent-multi-diff.t/B

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+I am file B.

unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent-multi-diff.t/C

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+I am file C.

unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent-multi-diff.t/dune-project

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+(lang dune 3.8)

unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent-multi-diff.t/run.t

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+The use of (concurrent ) illustrated in the context of diffing multiple files.
+
+Say we want to diff 3 files.
+  $ cat A
+  I am file A.
+  $ cat B
+  I am file B.
+  $ cat C
+  I am file C.
+
+We set up a (progn ) rule to diff all of them against their generated versions.
+
+  $ cat > dune << EOF
+  > (rule
+  >  (action
+  >   (progn
+  >    (with-outputs-to A.diff (echo "I am file A.\n"))
+  >    (with-outputs-to B.diff (echo "I am certainly file B.\n"))
+  >    (with-outputs-to C.diff (echo "I am most certainly file C.\n")))))
+  > 
+  > (rule
+  >  (action
+  >   (progn
+  >    (with-outputs-to some-target (echo a))
+  >    (diff A A.diff)
+  >    (diff B B.diff)
+  >    (diff C C.diff))))
+  > EOF
+
+We can now run the rule and see that we fail before diffing C.
+
+  $ dune build
+  File "B", line 1, characters 0-0:
+  Error: Files _build/default/B and _build/default/B.diff differ.
+  [1]
+
+We can check which diffs were run by asking Dune to promote the files.
+
+  $ dune promotion apply
+  Promoting _build/default/B.diff to B.
+
+Since we failed early, only B was promoted.
+
+Let's reset B to its original state.
+
+  $ rm B
+  $ cat > B << EOF
+  > I am file B.
+  > EOF
+
+If we implement the rule using (concurrent ) instead.
+
+  $ cat > dune << EOF
+  > (rule
+  >  (action
+  >   (progn
+  >    (with-outputs-to A.diff (echo "I am file A.\n"))
+  >    (with-outputs-to B.diff (echo "I am certainly file B.\n"))
+  >    (with-outputs-to C.diff (echo "I am most certainly file C.\n")))))
+  > 
+  > (rule
+  >  (action
+  >   (concurrent
+  >    (with-outputs-to some-target (echo a))
+  >    (diff A A.diff)
+  >    (diff B B.diff)
+  >    (diff C C.diff))))
+  > EOF
+
+We see that all the files get diffed.
+
+  $ dune build
+  File "B", line 1, characters 0-0:
+  Error: Files _build/default/B and _build/default/B.diff differ.
+  File "C", line 1, characters 0-0:
+  Error: Files _build/default/C and _build/default/C.diff differ.
+  [1]
+
+And we have promotions for the two that failed.
+
+  $ dune promote
+  Promoting _build/default/B.diff to B.
+  Promoting _build/default/C.diff to C.
+