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common-errors.md

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Common syntax errors in Clojure (v1.10)

Stack trace format

Errors manifest as Java stack traces. The format usually is following:

Exception in thread "main" Syntax error compiling at (foo.clj:7:1).
        at clojure.lang.Compiler.analyze(Compiler.java:6808)
        at clojure.lang.Compiler.analyze(Compiler.java:6745)
        ...
Caused by: java.lang.RuntimeException: No such var: s/foobar
        at clojure.lang.Util.runtimeException(Util.java:221)
        at clojure.lang.Compiler.resolveIn(Compiler.java:7387)
        ...

In the stack traces the most useful information usually is the error message of RuntimeException, here No such var: s/foobar.

On the first row, the row and column of the syntax error is mentioned. Here it is (foo.clj:7:1) which means first character on 7th row in foo.clj file.

RuntimeExceptions

  • Unmatched delimiter: )

    • One closing parenthesis too much

  • EOF while reading, starting at line 1

    • Missing closing parenthesis

  • java.lang.Long cannot be cast to clojure.lang.IFn

    • First symbol after parenthesis is not a function but a long.
    • If you want to create a list, use quote: '(1 2 3)

  • java.lang.String cannot be cast to clojure.lang.IFn

    • First symbol after parenthesis is not a function but a string.

  • Caused by: clojure.lang.ArityException: Wrong number of args (1) passed to: user/test

    • Wrong number of arguments given to a function.

  • Unable to resolve symbol: foo in this context

    • fsunction name not found

  • No such var: s/foo

    • The name s/foo is not bound to a variable or function.

  • Don't know how to create ISeq from: clojure.core$inc

    • Wrong order of arguments for e.g. map, filter or reduce

  • Unable to resolve symbol: bar' in this context

    • Single quotes used for a string instead of double quotes