Appendix C — Common mistakes and how to fix them

These are the places people often trip when writing Lowent with habits from other languages. The meanings of diagnostic codes are in Appendix B, and explanations are in the chapters named in the tables.

The surface#

Mistake	Diagnostic	Fix
Writing output before input	E-CLAUSE-ORDER	Follow the clause order table. Clauses other than inputs are moved by --fmt (chapter 3)
Opening a block with a period or line break, as in struct p .	E-STMT-NODO	struct p do … end .
Closing with ; or ,	E-VOCAB-REMOVED	The full stop .
Reading a field with p.x	E-FIELD-GLUED	field p. x .
for x in xs	E-VOCAB-REMOVED	for x xs. do … end .
let x f64 .5 .	E-LET-NOVALUE	0.5
Naming a local the same as a parameter	E-NAME-SHADOW	Pick a new name
Using names like len, max or text	E-NAME-BUILTIN and others	Builtins and the word that opens a text literal cannot be names
A module name equal to an op name	E-NAME-DUP	Rename the module
Leaving the stop off a variable, as in add a b .	E-DOT-MISSING	add a. b. . — a variable is written a. (chapter 3)
let x be u8 4 .	E-LET-BE	let x u8 4 .
One stop short, as in return add a. b. ., or no stop after end	E-DOT-MISSING	return add a. b. . . · end . — close as many as you opened
One stop too many, as in if gt a. 3 . . do … end .	E-CLOSER-EXTRA	if gt a. 3 . do … end .
for i count u64 n . do	E-FOR-OLD	repeat i u64 n. do

Table 50.1 — Mistakes on the surface

Values and flow#

Mistake	Diagnostic	Fix
set on a let	E-IMMUTABLE	Make it with var
Using an integer as a condition	E-TYPE-COND	Write the question, as in gt n. 0 .
Expecting overflow to wrap as in C	E-VM-OVERFLOW (at run time)	Choose the outcome with wrap_add, sat_add or chk_add
Expecting narrow u8 x. . to truncate	E-VM-CAST (at run time)	narrow_wrap, narrow_sat, narrow_try
guard’s else does not leave	E-GUARD-FALLTHROUGH	return, break, continue, panic, or use if
No return on some path	E-RETURN-PARTIAL	Return on every path
Using if as a value	E-IF-VALUE	Make a var and set it in each branch
Missing cases in match	E-MATCH-INEXHAUSTIVE	The missing case or case _
Writing enum variants one per line without periods	E-ENUM-DOT	red . for each variant
Extracting with some_value without checking	E-VM-NONE (at run time)	guard is_some · value_or · match
array u8 4	E-TYPE-ORDER	array 4 u8

Table 50.2 — Mistakes with values and flow

Effects, capabilities and memory#

Mistake	Diagnostic	Fix
Printing from a fn	E-EFFECT-CALC	proc with effects io
Writing effects none on a fn	E-EFFECT-REDUNDANT	Delete the clause
A fn writing to the caller through a mut parameter	E-EFFECT-PURITY	Make it a proc
effects io without receiving a capability	E-EFFECT-NO-CAP	input out cap io .
Receiving a capability but not passing it to the op that uses it	E-CAP-MISSING	As the first operand, as in stdio.write out. 1 …
The entry point receiving data	E-ENTRY-PARAMS	The entry point receives only capabilities. Arguments via cap args
Writing to a slice without mut	E-TYPE-MUT	mut slice
Writing through ref	E-TYPE-REF	mut_ref
Borrowing the same value for writing twice	E-EXCL	Split the borrows or split the order
Returning a reference to a local	E-ESCAPE	Return a value or store it in the caller’s storage
mut ref slice	E-MREF-SLICE	Return a new slice
Carrying a region’s bytes outside	E-REGION-ESCAPE	Widen the region or carry out only values
Not closing a file or buffer	E-OWN-INCOMPLETE	Call close or finish. To discard, drop
Using a moved value again	E-OWN-MOVED	Use it before handing over, or get it back
Using the heap on a target without an operating system	E-HEAP-NOHOST	A fixed window (cap allocator) and alloc

Table 50.3 — Mistakes with effects, capabilities and memory

Abstraction and concurrency#

Mistake	Diagnostic	Fix
Writing fn or proc in a trait signature	E-TRAIT-SIG	area input s self . output u64 .
Leaving the effects line off a proc	E-EFFECT-MISSING	Write effects <atoms> . — effects none . when there are none (--doc prints inferred:)
A type argument not satisfying the trait	E-BOUND-UNSAT	Add satisfies and the op
Giving a run-time value to a comptime input	E-COMPTIME-ARG	A literal or module constant
spawn <op> outside a task_group	E-SPAWN-SCOPE	Wrap it in task_group do … end .
Writing an accumulator in a split loop	E-PAR-CARRY	reduce <place> <op> .
Using a stage like sorting in pipe	E-PIPE-STAGE	Use only the seven stages and five terminals
Calling an effectful attached op with method inside a fn	(not rejected in this edition)	Call it directly as <type>.<name> (chapter 23)

Table 50.4 — Mistakes with abstraction and concurrency