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:

      (
SYNTAX ERROR
      (
      ∧

      'Hello world 
SYNTAX ERROR: Invalid token
      'Hello world
      ∧

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.

      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
        ∧

Depending on the value of fn, 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

SYNTAX ERROR

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