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
