refactor: Add Result.fromTry

(May or may not involve tricking the compiler)

Author: MarcellPerger1

src/main/java/net/marcellperger/mathexpr/util/ThrowingSupplier.java

@@ -0,0 +1,6 @@
+package net.marcellperger.mathexpr.util;
+
+@FunctionalInterface
+public interface ThrowingSupplier<T, E extends Throwable> {
+    T get() throws E;
+}

src/main/java/net/marcellperger/mathexpr/util/Util.java

@@ -257,4 +257,28 @@ public void forEachRemaining(Consumer<? super T> action) {
             return VoidVal.val();
         };
     }
+
+    // These 2 trick Java into throwing checked exceptions in an unchecked way
+    @Contract("_ -> fail")
+    public static <T> T throwAsUnchecked(Throwable exc) {
+        throwAs(exc);  // <RuntimeException> is inferred from no `throws` clause
+        // Java doesn't know that this always throws so this lets us
+        // do `return/throw throwAsUnchecked()` to make Java's flow control analyser happy
+        throw new AssertionError("Unreachable");
+    }
+    @SuppressWarnings("unchecked")
+    @Contract("_ -> fail")
+    public static <E extends Throwable> void throwAs(Throwable exc) throws E {
+        // We do a little type erasure hack to trick Java:
+        //  - E will be type-erased to Throwable so this will become
+        //       throw (Throwable)exc;
+        //     but exc is already Throwable due to the param type so
+        //     this is like a runtime no-op.
+        //  - But javac will see that we're throwing an E which is allowed!
+        //  - The reason we need a separate method is so that there is a
+        //     generic for javac to type-erase (otherwise JVM would check that
+        //     it's actually that concrete type when it is thrown and this way
+        //     it only checks for the base condition)
+        throw (E)exc;
+    }
 }

src/main/java/net/marcellperger/mathexpr/util/rs/Result.java

@@ -1,5 +1,6 @@
 package net.marcellperger.mathexpr.util.rs;
 
+import net.marcellperger.mathexpr.util.ThrowingSupplier;
 import net.marcellperger.mathexpr.util.Util;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -51,6 +52,19 @@ static <T, E> Result<T, E> newErr(E err) {
         return new Err<>(err);
     }
 
+    static <T, E extends Throwable> Result<T, E> fromTry(ThrowingSupplier<? extends T, E> inner, Class<E> catchThis) {
+        try {
+            return newOk(inner.get());
+        } catch (Throwable exc) {
+            try {
+                return newErr(catchThis.cast(exc));
+            } catch (ClassCastException c) {
+                // We've handled E, so only unchecked exceptions should reach here
+                // so it's safe to throw them (but Java doesn't know that so we trick it)
+                return Util.throwAsUnchecked(exc);
+            }
+        }
+    }
 
     default @Nullable Ok<T, E> ok() {
         return switch (this) {

src/test/java/net/marcellperger/mathexpr/util/rs/ResultTest.java

@@ -442,4 +442,41 @@ void errOption() {
         assertEquals(Option.newNone(), getOk().errOption());
         assertEquals(Option.newSome("TESTING_ERROR"), getErr().errOption());
     }
+
+    @SuppressWarnings("CommentedOutCode")
+    @Test
+    void fromTry() {
+        assertEquals(Result.newOk(314), Result.fromTry(() -> 314, CustomException.class));
+        CustomException ce = new CustomException("Unchecked (expected) exception");
+        CheckedCustomException cce = new CheckedCustomException("Checked (expected) exception");
+        assertEquals(Result.newErr(cce), Result.fromTry(() -> {throw cce;}, CheckedCustomException.class));
+        assertEquals(Result.newErr(ce), Result.fromTry(() -> {throw ce;}, CustomException.class));
+        UnexpectedCustomException uce = new UnexpectedCustomException("Unexpected unchecked exc");
+        assertThrows(UnexpectedCustomException.class, () -> Result.fromTry(() -> {throw uce;}, CustomException.class));
+        // This doesn't compile so GOOD! (How do I write a test that something DOESN'T compile???)
+        // UnexpectedCheckedCustomException ucc = new UnexpectedCheckedCustomException("Unexpected checked exc");
+        // assertThrows(UnexpectedCheckedCustomException.class, () -> Result.fromTry(() -> {throw ucc;}, CheckedCustomException.class));
+    }
+
+    static class UnexpectedCustomException extends RuntimeException {
+        public UnexpectedCustomException(String message) {
+            super(message);
+        }
+    }
+    static class CustomException extends RuntimeException {
+        public CustomException(String message) {
+            super(message);
+        }
+    }
+    static class CheckedCustomException extends Exception {
+        public CheckedCustomException(String message) {
+            super(message);
+        }
+    }
+    @SuppressWarnings("unused")  // used in the does-not-compile test
+    static class UnexpectedCheckedCustomException extends Exception {
+        public UnexpectedCheckedCustomException(String message) {
+            super(message);
+        }
+    }
 }