Added check for async result

Author: SamLaptop

gitbook/asyncResult/check.md

@@ -0,0 +1,76 @@
+# Result.check
+
+Namespace: `FsToolkit.ErrorHandling`
+
+The intent of check is to allow an Ok result value to be validated. 
+
+`check` takes a validation function of the form `'ok -> Async<Result<unit, 'error>>` and a result of the form `Async<Result<'ok, 'error>>`. 
+
+If the async-wrapped result is `Ok x` then the validation function is applied, and if the validation function returns an error, this new async-wrapped error is returned. Otherwise, the original async-wrapped `Ok x` result is returned. If the original async-wrapped result is an Error, the original async-wrapped result is returned.
+
+## Function Signature
+```fsharp
+('ok -> Async<Result<unit,'error>>) -> Async<Result<'ok,'error>> -> Async<Result<'ok,'error>>
+```
+
+## Examples
+
+### Example 1
+
+Given the following function that returns true for the id `123`
+```fsharp
+checkEnabled : int -> Async<bool>
+```
+
+```fsharp
+AsyncResult.ok (
+    {|
+        PolicyId = 123
+        AccessPolicyName = "UserCanAccessResource"
+    |}
+
+)
+|> AsyncResult.check (fun policy ->
+    asyncResult {
+        let! isEnabled = checkEnabled policy.PolicyId
+
+        return
+            if not isEnabled then
+                Error(
+                    $"The policy {policy.AccessPolicyName} cannot be used because its disabled."
+                )
+            else
+                Ok()
+
+    }
+)
+// AsyncResult.Ok {| AccessPolicyName = "UserCanAccessResource"; IsEnabled = true; |}
+```
+
+### Example 2
+
+```fsharp
+AsyncResult.ok (
+    {|
+        PolicyId = 456
+        AccessPolicyName = "UserCanAccessResource"
+    |}
+
+)
+|> AsyncResult.check (fun policy ->
+    asyncResult {
+        let! isEnabled = checkEnabled policy.PolicyId
+
+        return
+            if not isEnabled then
+                Error(
+                    $"The policy {policy.AccessPolicyName} cannot be used because its disabled."
+                )
+            else
+                Ok()
+
+    }
+)
+
+// AsyncResult.Error "The policy UserCanAccessResource cannot be used because its disabled."
+```