Document Catch (⎊)

1 files changed, 19 insertions, 1 deletions

diff --git a/doc/assert.md b/doc/assert.md
index 121d8a1e..f193be21 100644
--- a/doc/assert.md
+++ b/doc/assert.md
@@ -1,6 +1,10 @@
 *View this file with results and syntax highlighting [here](https://mlochbaum.github.io/BQN/doc/assert.html).*
 
-# Assert
+# Assert and Catch
+
+BQN provides some simple facilities for dealing with errors. Errors are an unusual sort of control flow; if possible, prefer to work with functions that return normally.
+
+## Assert
 
 BQN takes the position that errors exist to indicate exceptional conditions that the developer of a given program didn't expect. However, the types of errors that BQN naturally checks for, such as mismatched shapes in Couple (`≍`), aren't always enough to detect exceptional conditions. Issues like numeric values that don't make physical sense will slip right through. BQN makes it easy for a programmer to check for these sorts of problems by building in the primitive Assert, written `!`. This function checks whether `𝕩` matches `1`: if it does, then it does nothing and returns `𝕩`, and otherwise it gives an error.
 
@@ -23,3 +27,17 @@ Because the left argument to a function is always computed before the function i
 - Handle errors with ordinary if-then logic (perhaps using [control structures](control.md)). This is probably the best path for user-facing applications where displaying an error goes through the user interface.
 - Write a function `Message` to compute the message, and call `𝕨 Message⊸!⍟(1⊸≢) 𝕩` or similar instead of `!`.
 - If the error will be caught elsewhere in the program, use a closure for the message and evaluate it when caught. With a function `Message` as above, `message ! 𝕩` works, and `{…}˙⊸! 𝕩` is a convenient syntax for block functions.
+
+## Catch
+
+The `Catch` modifier allows you to handle errors in BQN (at present, it's the only way to do so). It evaluates the function `𝔽` normally. If this function completes without an error, Catch just returns that result. If not, it stops the error, and calls `𝔾` with the original arguments instead.
+
+        ⌽⎊'x' "abcd"  # No error
+
+        ⌽⎊'x' 2       # Can't reverse a unit
+
+        0.5 ⌽⎊⊣ ↕6    # A two-argument example
+
+Catch doesn't know anything about what an error *is*, just whether there was one or not. In fact, the idea of error message doesn't feature at all in core BQN: it's purely part of the language environment. So you need a system value to access information about the error. Right now the only one is `•CurrentError`, which is a function that returns a message for the error currently caught (if any).
+
+        ⌽⎊•CurrentError 2