swift - What's the difference between if nil != optional … and if let _ = optional … -

i need test if expression returns optional nil. seems no-brainer, here code.

if nil != self?.checklists.itempassingtest({ $0 === note.object }) {     … } 

which, reason, looks unpleasant eye.

if let item = self?.checklists.itempassingtest({ $0 === note.object }) {     … } 

looks better me, don't need item, need know if 1 returned. so, used following.

if let _ = self?.checklists.itempassingtest({ $0 === note.object }) {     … } 

am missing subtle here? think if nil != optional … , if let _ = optional … equivalent here.

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update address concerns in answers

1. i don't see difference between nil != var , var != nil, although use var != nil. in case, pushing != nil after block gets boolean compare of block mixed in boolean compare of if.

2. the use of wildcard pattern should not surprising or uncommon. used in tuples (x, _) = (10, 20), for-in loops for _ in 1...5, case statements case (_, 0):, , more (note: these examples taken swift programming language).

this question functional equivalency of 2 forms, not coding style choices. conversation can had on programmers.stackexchange.com.

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after time, swift 2.0 makes moot

if self?.checklists.contains({ $0 === note.object }) ?? false {     … } 

after optimization, 2 approaches same.

for example, in case, compiling both following swiftc -o -emit-assembly if_let.swift:

import darwin  // using arc4random ensures -o doesn’t // ignore if statement let i: int? = arc4random()%2 == 0 ? 2 : nil  if != nil {   println("set!") } 

vs

import darwin  let i: int? = arc4random()%2 == 0 ? 2 : nil  if let _ = {   println("set!") } 

produces identical assembly code:

    ; call arc4random     callq   _arc4random     ; check if lsb == 1      testb   $1, %al     ; if is, skip println     je  lbb0_1     movq    $0, __tv6if_let1igsqsi_(%rip)     movb    $1, __tv6if_let1igsqsi_+8(%rip)     jmp lbb0_3 lbb0_1:     movq    $2, __tv6if_let1igsqsi_(%rip)     movb    $0, __tv6if_let1igsqsi_+8(%rip)     leaq    l___unnamed_1(%rip), %rax  ; address of "set!" literal     movq    %rax, -40(%rbp)     movq    $4, -32(%rbp)     movq    $0, -24(%rbp)     movq    __tmdss@gotpcrel(%rip), %rsi     addq    $8, %rsi     leaq    -40(%rbp), %rdi     ; call println     callq   __tfss7printlnu__fq_t_ lbb0_3:     xorl    %eax, %eax     addq    $32, %rsp     popq    %rbx     popq    %r14     popq    %rbp     retq