Errors, exactly as the compiler says them

The error messages are part of the language. Each one below is what the compiler prints, and the site's tests fail if a message changes. Run a cell to see its output beside the expected one.

A string is not changed in place

Strings are immutable, as in Python; build a new one instead.

Viv [1]

s = "hi"
s[0] = "H"

expected error: 'str' object does not support item assignment…

deriv cannot differentiate a call to a def

Write the expression inline in the fn instead of calling f.

Viv [2]

def f(x) {
    return x * x
}
show deriv(fn(a) -> f(a), 1.0)

expected error: cannot differentiate through…

A def does not change a record it is given

A record is a value: change a copy and return it. An array inside it can still be written in place.

Viv [3]

record P { x: Num }
def f(p: P) {
    p.x = 2.0
    return p.x
}
show f(P { x: 1.0 })

expected error: `p` is a parameter, and a def does not change the values it is given…

Unknown function

Viv [4]

show frobnicate(1.0)

expected error: unknown function…

Arity is checked

Viv [5]

def f(a, b) {
    return a + b
}
show f(1.0)

expected error: expects 2 argument…

Out of bounds says where

Viv [6]

a = [1.0, 2.0]
show a[5]

expected error: index 5 out of bounds for dimension 0 (size 2)…

An index before the start stops

A negative index counts from the end, as in Python: -1 is the last element. Two elements have no element -3.

Viv [7]

a = [1.0, 2.0]
show a[-1]
show a[0.0 - 3.0]

expected error: index -3 out of bounds for dimension 0 (size 2)…

Shape mismatch

Viv [8]

show [1.0, 2.0] + [1.0, 2.0, 3.0]

expected error: array shape mismatch: [2] vs [3]…

break outside a loop

Viv [9]

break
show 1.0

expected error: `break`/`continue` outside a loop…

Undefined variable

Viv [10]

show nonexistent + 1.0

expected error: undefined variable…

plot wants arrays

Viv [11]

figure("Nothing to see", "x", "y")
plot(1.0, 2.0)

expected error: `plot` takes two arrays…

Keywords cannot name functions

record, test, show and the other keywords are reserved.

Viv [12]

def record(v) {
    return v
}
show record(1.0)

expected error: expected function name…

A number is not a condition

if and while take a Bool. Write the comparison you mean.

Viv [13]

count = 3.0
if count {
    show count
}

expected error: condition must be a boolean…

No arithmetic on a Bool

true is not 1. Every engine refuses this.

Viv [14]

x = true + 1.0

expected error: boolean…

An array is not a condition

A comparison of two arrays is a mask, not a Bool; the note says to write all(a == b).

Viv [15]

a = [1.0, 2.0]
if a == a {
    show 1.0
}

expected error: condition must be a boolean, and this is an array…

% by zero stops the program

/ by zero gives inf, as IEEE floats do, but % and // by zero have no answer and stop.

Viv [16]

show 7.0 % 0.0

expected error: division by zero in `a % b`…

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