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Fix errors

Read an L4 error message, find the cause, and fix it. The error message reference lists the errors one by one; this page shows how to work through them.

Read a diagnostic

Run l4 check myfile.l4, or open the file in VS Code, which shows the same messages as red underlines. Each problem is printed as a block:

File:     err-layout.l4
  Range:    4:1-4:1
  Source:   parser
  Severity: DiagnosticSeverity_Error
  Message:
      |
    4 | x TIMES 2
      | ^
    incorrect indentation (got 1, should be greater than 1)

Read three things, in this order:

  1. Range: line:column-line:column of the code at fault. Go there first.
  2. Source: parser means the file could not be read as L4 at all, usually because of layout or a misplaced keyword. check means it was read but the types do not fit together.
  3. Message: the explanation, in plain words. Read all of it before guessing.

When there are several errors, fix the first one and check again: later errors are often caused by the first. l4 check exits with 0 when the file is valid and 1 otherwise.

A debugging routine

  1. Run l4 check file.l4 after every few edits. It is fast because it evaluates nothing.
  2. Read the first diagnostic only. Note its source (parser or check) and go to its range.
  3. For a parser error, look at the line above as well: a layout problem is often caused by the construct before it.
  4. For a check error, find the pair "expected X … but is here of type Y" and decide which side is wrong.
  5. Still can't see it? Comment out half of the file (-- per line, or {- ... -} around a block), check again, and narrow it down.
  6. Search the error message reference for a phrase from the message.

Common errors

Indentation

L4 is layout-sensitive: the body of a definition must be indented further than the line that introduces it. The diagnostic above comes from this file:

GIVEN x IS A NUMBER
GIVETH A NUMBER
DECIDE `twice` IS
x TIMES 2

Indent the body ( x TIMES 2) or keep it on the same line. Any parser error that says "incorrect indentation" or "unexpected …" is usually a layout problem. See Incorrect indentation.

Type mismatch

The GIVETH type is a promise that the body has to keep:

GIVEN x IS A NUMBER
GIVETH A BOOLEAN
DECIDE `is large` IS x PLUS 1

The type of this definition must match its type signature at err-mismatch.l4:2:8-17, namely

  BOOLEAN

but is here of type

  NUMBER

Decide which side is wrong. Here it is the body: DECIDE `is large` IF x GREATER THAN 1000. L4 never converts types for you; when you need a conversion, write it (see Coercions). See Types.

Unknown name

A file defines `is an adult` for a Person, and a test misspells it as #EVAL `is an adutl` `Alice`:

I could not find a definition for the identifier

  `is an adutl`

which I have inferred to be of type:

  FUNCTION FROM Person TO res11

The second half shows how the unknown name is used: here, as a function from Person to something not yet known (res11). Use that to find the definition you meant. In order of likelihood:

  • Spelling, including spaces inside backticks: `is an adult` and `is an adult ` are different names.
  • Missing backticks: is an adult without backticks is three separate words.
  • The definition does not exist: order does not matter for top-level definitions, but the name must be defined in the file or in an imported module.

Missing IMPORT

A library name such as Date only exists after you import the library that defines it. Without IMPORT daydate, the line `the closing date` MEANS Date 15 1 2025 gives "I could not find a definition for the identifier Date", the same message as a typo. Add the import:

IMPORT daydate

`the closing date` MEANS Date 15 1 2025

#EVAL `the closing date`

If an unknown name is one you did not define yourself, check the library reference for the library that provides it. See I could not find a definition for the identifier and Module not found.

Ambiguous operator: mixed types in a comparison

Comparison operators such as AT LEAST work on numbers, strings and booleans, as long as both sides have the same type. Compare a number with a string and L4 cannot choose:

DECLARE Person
    HAS `name` IS A STRING
        `age`  IS A NUMBER

GIVEN person IS A Person
GIVETH A BOOLEAN
DECIDE `is an adult` IF
    person's `age` AT LEAST "18"

There are multiple definitions for the identifier

  `__GEQ__`

and I do not have sufficient information to make a choice between them.
The options are:

  `__GEQ__` (predefined) of type FUNCTION FROM NUMBER AND NUMBER TO BOOLEAN
  `__GEQ__` (predefined) of type FUNCTION FROM STRING AND STRING TO BOOLEAN
  `__GEQ__` (predefined) of type FUNCTION FROM BOOLEAN AND BOOLEAN TO BOOLEAN

__GEQ__ is the internal name of AT LEAST (>=). None of the listed versions takes a NUMBER and a STRING, which is the problem. Make both sides the same type: person's `age` AT LEAST 18.

The same "multiple definitions" message appears with __PLUS__ and other operators when an imported library adds versions of them (daydate adds PLUS for dates) and the operands do not show which one you mean. Fix a mistyped literal, or give one operand a known type.

@export with a function-typed input

A rule marked @export cannot take a function as an input, because a function cannot be sent as JSON. The error is "Function type inputs are not supported for @export". See ASSUME and @export.

Next steps