SYNTAX ERROR
A SYNTAX ERROR is an error message which indicates that a rule of APL syntax was violated. A SYNTAX ERROR may indicate an ill-formed expression which violates APL's grammar: <source lang=apl>
(
SYNTAX ERROR
(
∧
'Hello world
SYNTAX ERROR: Invalid token
'Hello world
∧
</syntaxhighlight> However, a SYNTAX ERROR may also indicate a problem not with the sequence of tokens in a statement, but with their meaning. For example, below an error results when a function is called with invalid valence: here, a monadic function with two arguments. <source lang=apl>
fn ← +
2 fn 3
5
fn ← +\ ⍝ A strictly monadic function
2 fn 3
SYNTAX ERROR: The function does not take a left argument
2 fn 3
∧
</syntaxhighlight> Depending on the value of <source lang=apl inline>fn</syntaxhighlight>, the same statement may or may not be considered invalid syntax. This is to be expected, since APL does not have a context-free grammar and needs to know the type of each variable to parse a statement.
| APL features [edit] | |
|---|---|
| Built-ins | Primitives (functions, operators) ∙ Quad name |
| Array model | Shape ∙ Rank ∙ Depth ∙ Bound ∙ Index (Indexing) ∙ Axis ∙ Ravel ∙ Ravel order ∙ Element ∙ Scalar ∙ Vector ∙ Matrix ∙ Simple scalar ∙ Simple array ∙ Nested array ∙ Cell ∙ Major cell ∙ Subarray ∙ Empty array ∙ Prototype |
| Data types | Number (Boolean, Complex number) ∙ Character (String) ∙ Box ∙ Namespace ∙ Function array |
| Concepts and paradigms | Conformability (Scalar extension, Leading axis agreement) ∙ Scalar function (Pervasion) ∙ Identity element ∙ Complex floor ∙ Array ordering (Total) ∙ Tacit programming (Function composition, Close composition) ∙ Glyph ∙ Leading axis theory ∙ Major cell search ∙ First-class function |
| Errors | LIMIT ERROR ∙ RANK ERROR ∙ SYNTAX ERROR ∙ DOMAIN ERROR ∙ LENGTH ERROR ∙ INDEX ERROR ∙ VALUE ERROR ∙ EVOLUTION ERROR |