Re: [Bug-apl] Bug in the parser?

[Juergen Sauermann]

Hi Elias, this is caused by an ambiguity of / (or ⌿, \. or ⍀ for that matter). These four APL symbols can, unfortunately,  be dyadic functions or monadic operators. Your example boils down to this:       A←1 2 3 4 5       B←0 1 1 0 1       A / ⍨ B The question is then should / be function - then we would have A (/⍨) B or operator - then we would have A / (⍨ B). Unfortunately there are cases where A is not yet knoen at the time this needs to be decided. I have read the IBM binding rules a number of times but they seem not to help. The problem of these rules is that they give different results in the cases where / is an operator and where / is a function. The binding rules say in which order tokens are bound but they do not decide if a token like / is a function or an operator. The solution in GNU APL is that / is assumed to be an operator unless the token left of it indicates a value (and then / is assumed to be a function. This happens at function definition time (where it is not yet known if A and B in the example above or m in your example) are values or functions). A closing ) left of / implies a value. You have already found the 2 ways to resolve this ambiguity: either put the left argument in parentheses (which marks it as a value in GNU APL) or to put the operator and its function argument in parentheses (which makes it a derived function). /// Jürgen On 11/25/2014 07:16 AM, Elias Mårtenson wrote: Given a typical implementation of prime-finding, the following _expression_ works fine:       (2 = +/[1] 0 = m ∘.| m) / m←⍳N 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 However, trying to swap the sides of the selection gives an error message:       m /⍨ 2 = +/[1] 0 = m ∘.| m←⍳N SYNTAX ERROR       m/⍨2=+/[1]0=m∘∘.∣m←⍳N       ^ ^ Adding a parentheses around the right-hand side does not work:       m /⍨ (2 = +/[1] 0 = m ∘.| m←⍳N) SYNTAX ERROR       m/⍨(2=+/[1]0=m∘∘.∣m←⍳N)       ^  ^ However, placing parens around the m makes it work:       (m) /⍨ (2 = +/[1] 0 = m ∘.| m←⍳N) 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 Another way of getting rid of the error is to place parens around the function:       m (/⍨) (2 = +/[1] 0 = m ∘.| m←⍳N) 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 This surely can't be right? Regards, Elias