Traps in writing Hilde

Every trap met in writing the compiler, the standard library or an application built on them, in one list: the error code and what the compiler said, the wrong and the right code, and a rule in one sentence. The line Now says what the compiler does about it now: the trap is removed, or the message names it and suggests the fix, or nothing changed and why. The line Test names the test that holds it. New entries go at the end.

Language and compiler

1. A linear value consumed in a branch that returns

• Error: H4004 Linearity Error, "The variable box is used inconsistently in the branches of an if statement."
• Wrong: if bad then destroy(x); return y; end if; and x used further down
• Right before: an else that holds the rest, or the decision taken before the if
• Now: removed. A branch that ends in a return on every path does not reach the statement after the if or the case, and its return has checked that every linear variable is consumed; the other branches alone decide the state after it.

2. An integer literal is an Int32 where its place asks for another type

• Errors: H3060 "Expected a value of type Nat64, but got a value of type Int32", H3003 "The type parameter T cannot be bound to both Int64 and Int32", H3027 "Cannot cast to Int64 from Int32", H3055 for the constructor
• Wrong: pages := 1; with a Nat64, insertBack(&!odd, 3) with a Buffer[Int64], 55 + idLength with an Index
• Right before: a typed constant, constant one: Nat64 := 1;
• Now: removed. A literal takes the type of the variable or path it is assigned to, of the other operand of an operator on either side, of a generic parameter that the other arguments bind, and of the type a constructor or generic call is to have; the assigned value of a constructor gets the type of its variable. A literal only stays an Int32 where nothing asks for a type. One thing is refused that was taken: 1 + n with n a Nat64 was an Int32 addition that narrowed n without a word, and is now a Nat64.
• Still: a literal assigned through a path of a reference, p->n := 0; with p: &![Counter, R] and n: Index, is H3060 (Int32, not Index); bind it first, let zero: Index := 0; p->n := zero;.

3. No operator precedence

• Error: H0005 Parse Error, "Error during parse."
• Wrong: a or b or c, start + 32 <= n, at + 826 * 86400
• Right: (a or b) or c, (start + 32) <= n, at + (826 * 86400)
• Now: a chain of one of the associative operators +, *, and and or parses and is grouped from the left; any other operator after an operation is H0009, which says that Hilde has no precedence and suggests the parentheses around the operation before the operator. Precedence itself stays out by design.

4. A local named like a declaration of the module

• Error: H6045 "The variable allowed collides with the name of a declaration."
• Rule: a local or parameter takes a name that no declaration of the module has.
• Now: unchanged; the message names the collision. A collision inside one module is one its writer sees and fixes in the same file.

5. A parameter named like an imported function

• Error: H3055 "start is generic but unconstrained", or H3050 for start + 1: the name meant the imported function
• Now: removed. A local or a parameter named like an imported declaration means the local inside its function. A refusal as for item 4 would break a module whenever another module it imports adds a function of the same name.

6. The interface imports what its contracts call

• Error: H6039 "I can't find a callable named length."
• Now: H6060 names the module that declares a function of that name, gives the import line, and says that an interface imports what its own contracts call.

7. A typeclass in a constraint that the module does not import

• Error: H9999 Internal Error, "No typeclass with the given name."
• Now: H3105, which names the typeclass and the module that declares it, with the import line.

8. An instance whose module is not imported

• Error: H3061 "the type Int64 does not implement the typeclass TotalOrder."
• Now: H3061 says, where an instance exists in a module that is not imported, which module holds it and how to import it.

9. No valueOr in the language

• Error: H6039 "I can't find a callable named valueOr."
• Rule: valueOr is a helper of single test files; a module that wants it declares it.
• Now: unchanged for a name no module declares; a name another module declares gets H6060 (item 6).

10. A contract that reads a byte of a String

• Error: H3098 "This condition calls Standard.String.nthByte -> ... -> @embed"
• Now: unchanged; the message names the chain of calls down to the C.

11. A field of a written reference, or a reborrow, used twice

• Error: H4011 "Cannot borrow ... because it is already consumed", "Cannot reborrow ... because it is already borrowed"
• Rule: &!(r->field) and a write reference passed on use the reference up; a borrow in an expression lasts to the end of its statement.
• Now: the rule stays, it is what linearity is; H4011 says it: for a write reference that is used up, a reborrow of its own for each use; for a borrow, one statement per borrow.

12. A borrowed literal and the apostrophe

• Errors: H0005 for get(&!cache, &1, 0); H0001 "Character not allowed" for '\''
• Now: '\'' is the apostrophe, 39. &1 is H0011, which says that only a variable is borrowed and shows the binding to write first.

13. A reserved word as a name

• Error: H0008, a parse error that names the word
• Now: H0008 suggests a free name for the words most often taken for one: from since, to until, body content, end finish, type kind, case choice, record entry, union variant.

14. An index on the result of a call

• Error: H0005
• Now: H0010 says that an index stands after a name and shows the binding to write first. Not removed: an index on a call would borrow a value that has no variable to live in.

15. An index of a module constant

• Error: H6039 "I can't find a variable named digits." with no place in the source
• Now: H6059 with the place: a path is read through a borrow, a constant cannot be borrowed, and the message shows the local to bind first.

16. A path into a value instead of a reference

• Error: H6043 "The head of a reference transform expression must be of a reference type, but I got Stamp"
• Now: H6043 shows the function over &[T, R] that returns &(r->slot).

17. A binding of a case that takes a name bound before

• Error: H6020 "push_var: var with the name stamp already exists"
• Now: H6020 says that a name stays bound to the end of its block, also after its value is consumed, and suggests another name.

18. A linear var assigned again, and None of a linear Option built early

• Errors: H4010 "Cannot assign to the variable result because it is not yet consumed."; H4012 "The variable nothing is not consumed by the time of the return statement."
• Rule: an Option of a linear type is linear, also as None, and a var of a linear type takes a new value only after the old one is consumed.
• Now: the rule stays; H4010 says it, and H4012 says it for a variable of an Option or Either.

19. A linear value in the branches of an if expression

• Error: H4005 "The variable plain is consumed multiple times within the same expression."
• Now: removed: one branch runs, so a value both branches consume once is consumed once. Found on the way: a value only one branch consumed was taken and left unconsumed on the other path; that is H4015 now.

20. A span of a borrow of a local bound with a region name

• Error: H3054 "The slot there should have type Span[Nat8, Region<5782>] but is declared to have type Span[Nat8, R]"
• Now: H3054 says, for a region of a borrow of a local, that it has no name in the source, and how to use the value instead.

21. Two modules export the same name

• Error: H2027 "Colliding imports"
• Now: H2027 shows the import with as.

22. \u{...} in a string literal

• Error: none; the six characters stayed in the string as written
• Now: \u{X} writes the code point X, one to six hexadecimal digits, as UTF-8; a code point above 10FFFF or a surrogate is H6058.

23. An imported C function inside @embed

• Error: gcc, "implicit declaration of function duckdb_close"
• Now: H3106: a call of a C name that the programme imports through pragma Foreign_Import, inside the text of an @embed, is refused with the Hilde name to call.

24. None() as an argument of a parameter of type Option

• Error: H3055 "None is generic but unconstrained"
• Wrong: second(box, None()) with a parameter choice: Option[Int32]
• Right: let none: Option[Int32] := None(); and second(box, none)
• Now: taken: an argument refused with H3055 is checked once more with the type of its parameter, when that type is written out or decided by the type expected of the call; so Right(right => None()) in a let of Either[Int32, Option[Int32]] is taken as well. A parameter of the callee's own type variable, as ignore(None()) or isEmpty(None()) over Option[T], still decides nothing and stays H3055.

Outside the compiler

25. Checks that name a generated C variable

• Rule: after a change to the names in the generated C, search scripts/ for the old names before the checks run again.
• Now: unchanged; it is a rule for whoever changes the code generator.

26. SQLite and SQLCipher in one programme

• Error: H2044 "This programme links both sqlite3 and sqlcipher"
• Now: unchanged; the message names both, and Standard.SqlCipher opens plain files with openPlainWriter and openPlainReader.

27. A setting of DuckDB that says yes to anything

• Rule: a protection is checked where it acts: Standard.DuckDB reads the settings back after the open.

28. A commit of DuckDB after a failed statement

• Rule: a database that aborts a transaction on the first error may still call its commit a success; Standard.DuckDB rolls back with an error.

29. A data: URL in a URL attribute of a template

• Effect: src="#ZgotmplZ" instead of the image
• Rule: in href, src and attributes with src, uri or url in the name, every scheme but http, https and mailto becomes #ZgotmplZ, by design; a value the server makes itself goes through |safe with the reason beside it.

30. A function of a newer Hilde in an application built with an older one

• Error: H2030 for a function the Hilde the application is built with does not have yet
• Rule: an application builds against the Hilde it is built with; what is missing there is missing for it.

31. An anchored pattern with a literal after its start

• Now: fixed in Standard.Regex: an anchored pattern skips the search for its required literal.

32. = between two spans

• Error: cc, "invalid operands to binary ==" on hilde_span_t
• Wrong: word = "export " with word: Span[Nat8, Static]
• Right: equal(word, "export ") with Standard.Text's equal
• Now: H3107: a comparison of spans, records or unions is refused by the type checker, H3108 where a call makes a type variable one of them.

33. A reserved word as a name: is, skip and the others

• Error: a parse error; at the time shown as H2017 for another module (34)
• Wrong: function is(found: Option[String], expected: Span[Nat8, Static]): Bool, let skip: Int32 := 3;
• Right: function holds(...), let skipCount: Int32 := 3;
• Now: H0008 names the word and suggests a free name, in the text and as a suggestion of the JSON form (skip gives skipCount, is gives holds).
• The reserved words, all of them: and, as, body, borrow, case, constant, do, else, end, ensures, false, for, from, function, generic, if, import, instance, invariant, is, let, method, module, nil, not, of, or, pragma, raise, raises, record, requires, return, sizeof, skip, then, to, true, type, typeclass, union, var, when, while, and the type words Free, Linear, Region, Type and Span (the syntax, section 1).

34. An unknown module for a parse error elsewhere

• Error: H2017 "Unknown module Standard.String" for Bindings.File
• Now: a module whose head cannot be read makes the order report its own parse error instead of leaving the modules unsorted.

35. A value made up to return after abort

• Error: H2031 "The function x does not end in a return statement." when a function ended in abort(...) and nothing after it
• Wrong: abort("bad"); return 0;, a value no caller ever sees
• Right: abort("bad"); as the last statement
• Now: removed. abort returns Never, and a call of a function that returns Never ends a path as a return does; 139 of the 160 values are gone.

36. A docstring between pragma Slot and its declaration

• Error: H0005 "Error during parse." at the docstring
• Wrong: pragma Slot;, then a docstring, then function sqliteClose(...)
• Right: the docstring before pragma Slot;, or a -- comment above the group of slots, which the contract check takes for each slot of the group; both check with exit 0
• Rule: nothing stands between pragma Slot; and the declaration it marks.
• Now: nothing changed: the parser still refuses a docstring there, and the contract check takes the comment above a group of slots for each of them.

37. A write reference handed on where a read reference is asked

• Error: H3060 "Expected a value of type &[Manifest, R], but got a value of type &![Manifest, Region<...>]."
• Wrong: peek(x) or peek(&~x) with x: &![T, R] and peek over &[T, R]
• Right: take the value and give it back, function set(m: Manifest, ...): Manifest, and inside borrow it for reading and for writing in turn; or give the function a write reference too, as Standard.Buffer.lengthMut does
• Now: nothing changed: a write reference is not read through by another function while it is held.

38. A field of a result borrowed in an ensures

• Error: H6043 "The head of a reference transform expression must be of a reference type" for ensures length(&(result.name)) = spanLength(name)
• Wrong: ensures length(&(result.name)) > 0;, a path into the value result
• Right: an accessor of the module that takes &result and gives what the condition needs, ensures nameLength(&result) = spanLength(name);; one that gives a span through asSpan is refused in a contract (H3098), one that gives a length is taken
• Now: the message names the form with a reference and a function, as trap 16 does; nothing else changed, since a contract reads its result as any expression reads a value.

39. A field of the result of a call

• Error: H0005 "Error during parse." at the dot, with no hint
• Wrong: nodeAt().kind, nth(xs, i).kind
• Right: let node: Node := nodeAt(); and then node.kind
• Now: H0012 says that a field stands after a name and shows the binding to write first, as H0010 does for an index. Not removed: the grammar takes a path only from a name.