# 4 Gotchas… - on avoiding pitfalls and confusion

    (ns gotchas
      [:require
        [scicloj.kindly.v4.kind :as k]
        [wolframite.api.v1 :as wl]
        [wolframite.flags :as flags]
        [wolframite.wolfram :as w :refer :all
         :exclude [* + - -> / < <= = == > >= fn
                   Byte Character Integer Number Short String Thread]]])

    (wl/start!)

    {:status :ok, :wolfram-version 14.2}

## 4.1 What not to do when using Wolfram via Wolframite

### 4.1.1 Don’t: mix up `wl/!` and `w/!` 😅

Make sure not to use `w/!` (logical negation in Wolfram) when you actually want to evaluate your expression with `wl/!`.

### 4.1.2 Don’t: Transfer huge data unnecessarily

By default, `(wl/! <expr>)` will transfer the return value back to Clojure side and turn it into Clojure data. You don’t want to do that if the data is “big”. Wolframite will try to detect large data and will return `:wolframite/large-data` instead of the actual result in such a case. If you really want the actual result then set the flag

    flags/allow-large-data

    :allow-large-data

Do: Keep the data on Wolfram-side, assigning it to a symbol.

Example:

    (def csv "Make the Wolfram-side symbol easier to use in Clojure" 'csv)

    (wl/! (w/do  (w/= csv "some really big value, read from a file...")
                 (w/Length csv)))

    0

We use `w/=` to assign the value to a Wolfram-side name, so that we can use it in subsequent expressions. Notice that `=` also returns the value, and Wolframite would normally turn that into `:wolframite/large-data`. But in this case, we want instead to see how long the data is.

## 4.2 Language differences between Wolfram and Wolframite

Although we try to avoid such things, sometimes, when you’re fighting the host language, it’s just not practical to carry over the original conventions. Here we will try to keep an up-to-date list of possible surprises, when coming from Wolfram to Wolframite (that are not easy to ‘fix’).

### 4.2.1 Usage

Different output display in Wolframite and Wolfram - Wolfram has different display forms and thus what you get to see in Wolframite (the Input Form) may differ from what you would see in the Wolfram Engine CLI or a notebook (typically, the Output or Standard Form).

### 4.2.2 Function names

`^` (`Power`) - Despite allowing a wider range of characters, we cannot use `^` in Wolframite because it is reserved for adding metadata to virtually all Clojure symbols. `(^ x 2)` will probably not do what you expect! Thus we use `(`w/`** 'x 2)` instead.

`:=` (`SetDelayed`) - Is actually `_=` in Wolframite. This is because `:` is usually a reserved in Clojure for creating keywords. It’s worth noting though that this is possibly nicer, in a way, because `_` looks like a placeholder, and defining a placeholder for the expression is what it does. It also sort of implies a delay…

`:>` (`RuleDelayed`) - Is `_>` in Wolframite.

`.` (`Dot`) - Has been changed to `<*>`, in the spirit of the inner product, because `.` is a key character in Clojure, and most object-oriented systems, for namespacing.

`/.` (`ReplaceAll`) - Similarly, has been changed to `x>>` (and `Replace` to `x>`) for consistency.

`=.` (`Unset`) - has also been changed to `=!`; be careful not to confuse this with `!=`!

### 4.2.3 Other

Symbols within threading macros. - After spending so much time playing with symbols, be careful of slipping into things like `(-> 'x '(Power 1))` This will not work because `'(Power 1)` is not evaluated, and so will be treated like any other symbol. However, if you use `w/Power` and friends then you do not have this problem.

Symbols passed to Wolfram must be alphanumeric - In the end, when they get passed to the Wolfram kernel, symbols must be strictly alphanumeric (apart from forward slashes and dollar signs), i.e. `r_2` is currently not allowed. This is due to underlying limitations of the Wolfram language. Much like with Mathematica however, we can get around this in general by using Wolframite’s aliasing system (see the relevant tutorials).

Vectors, `[]`, vs lists, `()`. - Lists are used to represent function calls and so when combining Clojure and Wolfram expressions, make sure that data literals are vectors. For example, `(wh/show (w/ListLinePlot (range 10)))` will fail (otherwise unexpectedly), but `(wh/show (w/ListLinePlot (into [] (range 10))))` will give you what you expect.

## 4.3 Wolframite quirks

### 4.3.1 Limitations of `w/fn`

- No destructuring (yet)

    source: notebooks/gotchas.clj