How many implicit parameters can be passed to a function?

Question

Can I pass as many implicit parameters to any Scala function or is there any limitation on it ?

Answer 1

The number of method parameters is limited to 255 by the definition of a method descriptor (§4.3.3), where the limit includes one unit for this in the case of instance or interface method invocations.

https://docs.oracle.com/javase/specs/jvms/se12/html/jvms-4.html#jvms-4.11

Answer 2

I have tried to define the following trait Function255:

trait Function255[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, -T19, -T20, -T21, -T22, -T23, -T24, -T25, -T26, -T27, -T28, -T29, -T30, -T31, -T32, -T33, -T34, -T35, -T36, -T37, -T38, -T39, -T40, -T41, -T42, -T43, -T44, -T45, -T46, -T47, -T48, -T49, -T50, -T51, -T52, -T53, -T54, -T55, -T56, -T57, -T58, -T59, -T60, -T61, -T62, -T63, -T64, -T65, -T66, -T67, -T68, -T69, -T70, -T71, -T72, -T73, -T74, -T75, -T76, -T77, -T78, -T79, -T80, -T81, -T82, -T83, -T84, -T85, -T86, -T87, -T88, -T89, -T90, -T91, -T92, -T93, -T94, -T95, -T96, -T97, -T98, -T99, -T100, T101, T102, T103, T104, T105, T106, T107, T108, T109, T110, T111, T112, T113, T114, T115, T116, T117, T118, T119, T120, T121, T122, T123, T124, T125, T126, T127, T128, T129, T130, T131, T132, T133, T134, T135, T136, T137, T138, T139, T140, T141, T142, T143, T144, T145, T146, T147, T148, T149, T150, T151, T152, T153, T154, T155, T156, T157, T158, T159, T160, T161, T162, T163, T164, T165, T166, T167, T168, T169, T170, T171, T172, T173, T174, T175, T176, T177, T178, T179, T180, T181, T182, T183, T184, T185, T186, T187, T188, T189, T190, T191, T192, T193, T194, T195, T196, T197, T198, T199, T200, T201, T202, T203, T204, T205, T206, T207, T208, T209, T210, T211, T212, T213, T214, T215, T216, T217, T218, T219, T220, T221, T222, T223, T224, T225, T226, T227, T228, T229, T230, T231, T232, T233, T234, T235, T236, T237, T238, T239, T240, T241, T242, T243, T244, T245, T246, T247, T248, T249, T250, T251, T252, T253, T254, T255, +R] extends AnyRef { self =>
  def apply(v1: T1, v2: T2, v3: T3, v4: T4, v5: T5, v6: T6, v7: T7, v8: T8, v9: T9, v10: T10, v11: T11, v12: T12, v13: T13, v14: T14, v15: T15, v16: T16, v17: T17, v18: T18, v19: T19, v20: T20, v21: T21, v22: T22, v23: T23, v24: T24, v25: T25, v26: T26, v27: T27, v28: T28, v29: T29, v30: T30, v31: T31, v32: T32, v33: T33, v34: T34, v35: T35, v36: T36, v37: T37, v38: T38, v39: T39, v40: T40, v41: T41, v42: T42, v43: T43, v44: T44, v45: T45, v46: T46, v47: T47, v48: T48, v49: T49, v50: T50, v51: T51, v52: T52, v53: T53, v54: T54, v55: T55, v56: T56, v57: T57, v58: T58, v59: T59, v60: T60, v61: T61, v62: T62, v63: T63, v64: T64, v65: T65, v66: T66, v67: T67, v68: T68, v69: T69, v70: T70, v71: T71, v72: T72, v73: T73, v74: T74, v75: T75, v76: T76, v77: T77, v78: T78, v79: T79, v80: T80, v81: T81, v82: T82, v83: T83, v84: T84, v85: T85, v86: T86, v87: T87, v88: T88, v89: T89, v90: T90, v91: T91, v92: T92, v93: T93, v94: T94, v95: T95, v96: T96, v97: T97, v98: T98, v99: T99, v100: T100, v101: T101, v102: T102, v103: T103, v104: T104, v105: T105, v106: T106, v107: T107, v108: T108, v109: T109, v110: T110, v111: T111, v112: T112, v113: T113, v114: T114, v115: T115, v116: T116, v117: T117, v118: T118, v119: T119, v120: T120, v121: T121, v122: T122, v123: T123, v124: T124, v125: T125, v126: T126, v127: T127, v128: T128, v129: T129, v130: T130, v131: T131, v132: T132, v133: T133, v134: T134, v135: T135, v136: T136, v137: T137, v138: T138, v139: T139, v140: T140, v141: T141, v142: T142, v143: T143, v144: T144, v145: T145, v146: T146, v147: T147, v148: T148, v149: T149, v150: T150, v151: T151, v152: T152, v153: T153, v154: T154, v155: T155, v156: T156, v157: T157, v158: T158, v159: T159, v160: T160, v161: T161, v162: T162, v163: T163, v164: T164, v165: T165, v166: T166, v167: T167, v168: T168, v169: T169, v170: T170, v171: T171, v172: T172, v173: T173, v174: T174, v175: T175, v176: T176, v177: T177, v178: T178, v179: T179, v180: T180, v181: T181, v182: T182, v183: T183, v184: T184, v185: T185, v186: T186, v187: T187, v188: T188, v189: T189, v190: T190, v191: T191, v192: T192, v193: T193, v194: T194, v195: T195, v196: T196, v197: T197, v198: T198, v199: T199, v200: T200, v201: T201, v202: T202, v203: T203, v204: T204, v205: T205, v206: T206, v207: T207, v208: T208, v209: T209, v210: T210, v211: T211, v212: T212, v213: T213, v214: T214, v215: T215, v216: T216, v217: T217, v218: T218, v219: T219, v220: T220, v221: T221, v222: T222, v223: T223, v224: T224, v225: T225, v226: T226, v227: T227, v228: T228, v229: T229, v230: T230, v231: T231, v232: T232, v233: T233, v234: T234, v235: T235, v236: T236, v237: T237, v238: T238, v239: T239, v240: T240, v241: T241, v242: T242, v243: T243, v244: T244, v245: T245, v246: T246, v247: T247, v248: T248, v249: T249, v250: T250, v251: T251, v252: T252, v253: T253, v254: T254, v255: T255): R
  override def toString(): String = "<function255>"
}

and the compiler complains with

Platform restriction: a parameter list's length cannot exceed 254.

so it seems the limit is 254. Indeed trait Function254 compiles fine.

Having the parameters implicit like so

def apply(implicit v1: T1, v2: T2, ...

makes no difference, 254 still compiles fine.

---

Applying Dmytro's comment, in Scala 3 the limit seems to be dropped by Drop function 22 limit #1758:

The limits of 22 for the maximal number of parameters of function types and the maximal number of fields in tuple types have been dropped.

Functions can now have an arbitrary number of parameters. Functions beyond Function22 are erased to a new trait scala.FunctionXXL and tuples beyond Tuple22 are erased to a new trait scala.TupleXXL. Both of these are implemented using arrays.