Add docs

src/op.nim

@@ -1,5 +1,15 @@
+## This module contains a generic type that can be used as a return type
+## for operations that could fail. It adds additional messages to the result.
+##
+## OP stands for "Operation result".
+
 type
   OP*[T] = object of RootObj
+    ## Object to wrap the result of an operation
+    ##
+    ## - `isOk`: Indicates if the operation was successful
+    ## - `val`: If successful, this will hold the real result value
+    ## - `error`: Otherwise this will hold an error message
     case isOk*: bool
     of true:
       val*: T
@@ -7,11 +17,31 @@ type
       error*: string
 
 proc ok*[T](val: T): OP[T] =
+  ## Wraps the given value in a successful operation result.
   OP[T](isOK: true, val: val)
 
 proc fail*(op: OP, msg: string): OP =
+  ## Will create a new operation result with the given error message.
+  ## The type for the operation result is taken from the `op` argument.
+  runnableExamples:
+    proc someProc(): OP[int] =
+      result.fail "Not implemented!"
+
+    let data = someProc()
+    assert data.isOk == false
+    assert data.error == "Not implemented!"
   OP(isOK: false, error: msg)
 
 proc fail*[T](msg: string): OP[T] =
+  ## Will create a new operation result with the given error message.
+  ## The type for the operation result is given explicitly.
+  ##
+  ## **See Also:**
+  ## - `fail proc
+  ##   <#fail,OP,string>`_
+  runnableExamples:
+    let res = fail[seq[float]] "Something is wrong!"
+    assert res.isOk == false
+    assert res.error == "Something is wrong!"
   OP[T](isOK: false, error: msg)