A collection of errors and many silly mistakes to remind myself what I did to fix them next time.

gremlins.clj

; java.lang.RuntimeException: Unable to resolve symbol: catch in this context, compiling: <file>:<line>:<column>

; catch` is not a function or macro, it's special syntax only understood in the context of the special form `try`. You can't do this without writing your own form of try (see try+ in slingshot for an example of that). 
; https://groups.google.com/forum/#!topic/clojure/4EN7RDbKqVE

; example - but still doesn't work in my code 
; https://github.com/scgilardi/slingshot

(ns math.expression
  (:require [tensor.parse]
            [clojure.tools.logging :as log])
  (:use [slingshot.slingshot :only [throw+ try+]]))

(defn read-file [file]
  (try+
    [...]
    (tensor.parse/parse-tree tree)
    [...]
    (catch [:type :tensor.parse/bad-tree] {:keys [tree hint]}
      (log/error "failed to parse tensor" tree "with hint" hint)
      (throw+))
    (catch Object _
      (log/error (:throwable &throw-context) "unexpected error")
      (throw+))))

; SOLUTION: 

; My indentation was wrong. If should nest in the try! *blush*

(try+)
...
(catch Exception e)

; should be 

(try+
  ...
  catch Exception e)

; Whoops! I never noticed it with the transaction in between

;----------

; java.lang.IllegalStateException: :db.error/cas-failed Compare failed: Something

(ns awesome.core
  (:require [datomic.api :as d])

(defn update-entity
  [db params]
  (let [conn awesome.db.datomic/conn
        entity-id (Long/parseLong (:entity-id params))
        entity-name (:entity-name params)
        entity (get-entity db entity-id)
        txn [[:db.fn/cas entity-id :entity/name (:entity/name entity) entity-name]]] ;<--- I think it is failing here.
    (@(d/transact conn txn))))

; SOLUTION:
  
; 1. I am using the wrong attribute in `entity`. Should just be :name as it is a mapped entity.
; 2. Check that the correct entity name is referenced.
  
; https://docs.datomic.com/on-prem/transactions.html#dbfn-cas
  
;----------  
  
; Wrong number of args (0) passed to: PersistentArrayMap
  
; SOLUTION:
  
; WAITING - Means what it says but I have no idea what I was doing wrong

;----------  
  
; java.lang.RuntimeException: Map literal must contain an even number of forms
  
; Make sure all values have keys
  
; datomic.query.EntityMap cannot be cast to clojure.lang.IAtom
; I think it is because I am trying to save an entire entity
  
  (defn update-entity
  [params]
  (let [conn awesome.db.datomic/conn
        db (d/db conn)
        entity-id (Long/parseLong (:server-id params))
        entity (d/entity db entity-id)
        new-entity entity]
    (swap! new-entity #(-> %
                           (update :entity/name (:entity-name params))
                           (update :entity/email (:entity-email params))))
    (:db-after @(d/transact conn [:db.fn/cas entity-id :entity entity new-entity]))))
  
 ; SOLUTION: use db.fn/cas 
  
 ; (compare-and-swap) function takes four arguments: an entity id, an attribute, an old value, and a new value.
 ; htps://docs.datomic.com/on-prem/transactions.html#dbfn-cas
  
(defn update-entity
  [params]
  (let [conn awesome.db.datomic/conn
        db (d/db conn)
        entity-id (Long/parseLong (:entity-id params))
        entity (get-entity db entity-id)]
    {:db-after @(d/transact conn
                            [[:db.fn/cas (:id entity) :entity/name (:name entity) (:entity-name params)]
                             [:db.fn/cas (:user-id entity) :user/email (:user-email entity) (:user-email params)]])}))