Re: [Bug-apl] Subtle parsing problem

[Juergen Sauermann]

Hi Elias,

this is because your lambdas are niladic. The right lambda is called before SEL,
while the left lambda is called by SEL. From SEL's perspective, 'then'; is a function
while 'else' is a value.

/// Jürgen


On 07/29/2014 07:21 AM, Elias Mårtenson wrote:

I was writing an operator that acts as an "if"-statement, calling one of two functions depending on the value of the argument:

∇Z ← (then SEL else) arg

  →(arg=1)/do¯then

  →(arg=0)/do¯else

  ⎕ES 'Illegal value for arg'

  →0

do¯then:

  Z ← then arg

  →0

do¯else:

  Z ← else arg

∇

Note that the then and else functions are called with an argument "arg". When I call this operator with two nihilic lambda functions, I get very strange behaviour:

      ⊣ ({⎕←'was true'} SEL {⎕←'was false'}) 0

was false

      ⊣ ({⎕←'was true'} SEL {⎕←'was false'}) 1

was false

was true

I would expect to get an error message here, or perhaps seeing (⎕NC '⍵') to be 0. I certainly didn't expect to see both functions be called. My suspicion is that there is a problem with the parser somewhere, but I think Jürgen will have to look at this one.

Regards,

Elias