typecheck: validate simple function returns

README.md

@@ -162,6 +162,17 @@ Function square only accepts 1 argument, but this call gives 2. Remove the extra
 Function squre is not known yet. Check the function name or add TEACH squre before calling it.
 ```
 
+Simple functions can also declare a return type. `claro typecheck` checks each `RETURN` expression against it:
+
+```claro
+TEACH square amount RETURNS NUMBER
+    CHECK TYPE amount IS NUMBER
+    RETURN amount
+END
+```
+
+Returning text from this function reports: `Type mismatch for return from square: expected NUMBER, but this value looks like TEXT.`
+
 The unknown-function diagnostic is validated for both modern `DO squre 4` and compatibility `CALL squre WITH 4` calls. Compatibility calls that leave `WITH` empty, such as `CALL greet WITH`, now count as zero arguments, so learners get the same missing-argument guidance as `DO greet` instead of the checker treating the blank as an argument.
 
 For functions with more than one checked parameter, Claro reports each mismatched argument with the parameter name: