Why can't I call load() on a variable? - empirical-lang

Question

I can call load() in the REPL.
>>> load("trades.csv")
symbol timestamp price size
AAPL 2019-05-01 09:30:00.578802 210.5200 780
AAPL 2019-05-01 09:30:00.580485 210.8100 390
BAC 2019-05-01 09:30:00.629205 30.2500 510
CVX 2019-05-01 09:30:00.944122 117.8000 5860
AAPL 2019-05-01 09:30:01.002405 211.1300 320
AAPL 2019-05-01 09:30:01.066917 211.1186 310
AAPL 2019-05-01 09:30:01.118968 211.0000 730
BAC 2019-05-01 09:30:01.186416 30.2450 380
... ... ... ...
But I can't load a command-line argument in a script:
let trades = load(argv[1])
I get an error:
Error: macro parameter filename requires a comptime literal
Error: unable to determine type for trades

Answer 1

Empirical is statically typed. load() will infer a Dataframe's type if the file path can be determined at compile time. Variables and arbitrary expressions are permitted. For example:
let filename = "trades.csv"
let trades = load("./" + filename)
The filename above is a constant (immutable) because it was declared with let. But argv is mutable (not constant) because it is implicitly declared with var.
Empirical must know the type of the Dataframe, which we will have to pass explicitly in this case. Fortunately, we can get a hint in the REPL.
>>> columns(load("trades.csv"))
symbol: String
timestamp: Timestamp
price: Float64
size: Int64
We will also have to call csv_load() instead, since that takes a template. Here is the script as it should be:
data Trade:
symbol: String,
timestamp: Timestamp,
price: Float64,
size: Int64
end
let trades = csv_load{Trade}(argv[1])
This compiles as expected. Calling print(trades) in the script will display the Dataframe.