# \#arity

**URL:** https://swi-prolog.discourse.group/tag/arity/133.md

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## [On type error of \`arg(1, a, \_)\`](https://swi-prolog.discourse.group/t/on-type-error-of-arg-1-a/5912)

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**Author:** [@kuniaki.mukai](https://swi-prolog.discourse.group/u/kuniaki.mukai)\
**Replies:** 2\
**Last updated:** [November 6, 2022, 1:33pm UTC](https://swi-prolog.discourse.group/t/on-type-error-of-arg-1-a/5912 "2022-11-06T13:33:38Z")

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I have found an uxpected type error. ?- arg(1, a, X). ERROR: Type error: \`compound' expected, found \`a' (an atom) ERROR: In: ERROR: \[10\] arg(1,a,\_102) ERROR: \[9\] toplevel\_call('\<garbage\_collected\>') at /Users/canto…