Gotchas

The things most likely to surprise you, especially coming from Python or C. Each links to the chapter that explains it.

Names#

You cannot reuse a name in an inner scope. A local or parameter cannot take the name of another visible local, a parameter, an import or a top-level declaration of the file. Python allows this; Base reports "already in scope". Syntax
An import takes its name. After import c, a parameter or local named c is an error, and the same goes for every imported module name.
Some ordinary-looking words are reserved: local, type, test, in, with, new, free, none. Syntax
The built-in function and type names cannot be declared: let format = ..., func print(...) and a parameter named str are errors. A struct field may use them.
A file cannot be named after a standard module: c.lucb, io.lucb, memory.lucb, os.lucb, strings.lucb and the others are refused.
A file name must be a valid identifier: color_space.lucb, not color-space.lucb.
A package is not a module. import luce_std is an error; import one of its modules: from luce_std import files. Modules and packages
A package called luce-std is written luce_std in code.

Tabs are errors; indent with four spaces. Syntax
Assignment is a statement, not an expression, and there is no ++ or --. Use while let where C would assign in a condition. Expressions
Comparisons do not chain. a < b < c is not a < b and b < c.
not binds tightly: not x == 5 means (not x) == 5. Write x != 5. Expressions
Conditions must be bool. if count: and if pointer: are errors.
& binds more tightly than ==, unlike C: flags & mask == 0 is (flags & mask) == 0, which is usually what you meant.

Overflow traps, in every build. +, - and * on integers stop the program when the result does not fit, including in --release. Use +% to wrap, +| to saturate, +? to get an optional. Expressions
// rounds toward zero, as C does: -7 // 2 is -3, where Python gives -4. % matches: -7 % 2 is -1.
/ is only for floats. Integers divide with //.
u8(x) and (u8)x differ. u8(300) traps; (u8)300 is 44. Expressions
Literal arithmetic is done in the type it is assigned to: let x: u8 = 200 + 100 traps, because 300 does not fit a u8. In a cast, (u8)(200 + 100), the sum is computed as i64 first and gives 44.
Integers and floats never convert by themselves. Write f64(count).
Atomic arithmetic wraps instead of trapping, as C's does. Concurrency

length counts bytes, not characters: "café".length is 5. Types
A str cannot be indexed by position, and there is no + to join strings. Use strings.format into a buffer, or luce-std's collections.Buffer.
A formatted string is not a value: let s = f"..." is an error. It goes to print, strings.format, a Writer or a fmt parameter. Types
A format specification takes a number or text: {n:x} and {name:>8} work, but a value that shows itself through display takes none; write {value}.
print and C's printf can appear out of order, because printf is buffered. C
main(arguments: str[]) traps on an argument that is not UTF-8. Take c.str[] for file paths. Functions

Structs are copied on assignment, not shared as Python objects are. Pass a pointer to share one. Structs
Arrays nest as in C: u8[3][2] is three arrays of two bytes. Types
A pointer to an array is indexed through *: with p: u8[4]*, write (*p)[1]. p[1] indexes the pointer itself. Memory
An optional must be unwrapped before use, including an optional pointer: node.next.value does not compile when next is Node*?. Errors
A match on an integer, character or string needs a _ arm, and so does a match on an integer-backed enum, since a combination of flags is not a case. Control flow
none cannot be an enum case, since .none means an empty optional.
A union has no constructor. Declare it and assign a member. Unions
The vector broadcast f32[4](x) needs the type written out; it does not work through a type alias. Types
hash values change between runs; do not store them.

A method that changes its receiver needs a changeable one: calling it on a let is an error, and the message names the method. Nothing marks such a method; its body does. Functions
A static func is called through the type: Point.origin(), not p.origin(). A function in a type without static is a method, even when it never uses self. Functions
Default argument values must be constants.
Lambdas cannot capture local variables. Pass state as a parameter or a context pointer. Functions
A generic function can only use what its constraints promise. var x: T with no value is an error for an unconstrained T. Generics

Every fallible call must be handled with try, catch or else; it cannot be ignored, even with _ = .... Errors
A catch handler's recover value must have the type of the expression. For a call that returns nothing, the handler needs no recover.
One try covers the whole expression after it, every fallible call inside it.
A caught failure's message lasts until its handler finishes. Read it, pass it on or raise it again inside the handler; to keep it, store strings.copy(failure.message). Errors
A trap cannot be caught, and defer does not run on a trap.
defer sees variables as they are when it runs, at the end of the block, not when it was written (unlike Go's argument evaluation). Control flow

Memory comes from the current allocator, which with changes. A type that allocates must free with free(...) in self.allocator, not the current one. Memory
Each new thread starts with memory.heap as its current allocator, not its parent's. Concurrency
Use after free and double free are not detected in a normal build. Build with --profile diagnostic, or AddressSanitizer through the C backend, to find them. Memory

luce-base test runs the tests of one package: the file's and those of the modules of its package it imports, but never a dependency's; with --package, as luc test runs it, every module of the package's too. Testing
A test that fails by an error or a false assert in its own body is reported and the run continues; any other trap, an assert in a function it calls included, ends the run.
--lib -o name writes name.a and name.h: pass -o libname, not -o libname.a. C
-W turns warnings on; without it, unused code is not reported. check -W exits with status 1 if it warned. Building