metosin/malli

mx/defn and malli.destructure/parse unexpected handling of seqexps

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#1005 opened on Feb 1, 2024

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Description

If I write

(require '[malli.experimental :as mx])
(require '[malli.core :as m])

(mx/defn ^:malli/always f [xs :- [:* :int]]
  (malli.core/validate [:* :int] xs))

(f 1)
;; => false

(f [1])
;; => Execution error (ExceptionInfo) at malli.core/-exception (core.cljc:136).
;;    :malli.core/invalid-input

(m/validate [:* :int] [1])
;; => true

I would expect (f 1) to throw an except and (f [1]) to return true.

I think this is because the schemas for the arguments are combined using [:cat ...] and [:cat [:* :int]] 'flattens' the expression, so [[1]] is not valid for [:cat [:* :int]]. In this simple case the args schema should be something like [:tuple [:* :int]] for which [[1]] is valid.

Of course this is not so simple for varargs.

Contributor guide

Tech stack
None
Domain
backendapi
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Clojuremalli basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
The issue involves mx/defn incorrectly handling seqexps in argument schemas. The problem seems to stem from using `:cat` combinator which flattens the `:*` schema. Investigation should focus on how argument schemas are composed in the mx/defn macro and how seqexps are treated. Relevant code likely lives in the mall core namespace, particularly the schema validation logic. The issue mentions that using `:tuple` instead of `:cat` might work for simple cases, but varargs require a more general solution.