Style

These rules apply to BPL in tests, examples, documentation and libraries.

Use lowercase j in complex literals throughout tests and examples, including adapted reference cases. Reserve uppercase J for explicit input-alias tests. Keep archived upstream source unchanged. Write literal matrices in array notation, [10 20 30⋄40 50 60], not as a reshape. Keep ⍴ where the example is about reshape. A ⋄ that separates rows has no spaces around it. A ⋄ that separates statements has a space on each side.

Brackets are the usual way to write a list, and to build an array from names or expressions. A bare literal list, such as 1 2 3, is a shorthand. Write a list of names in brackets, as in [a b], even where a b would also work. Write [n 2]⍴0, [x],y, [a⋄b] and f⍤1 m, not APL’s (n,2)⍴0, (⊂x),y, ⊃[a b] and ⊃f¨↓m. When no item of a bracket list contains a space, separate the items with spaces, not ;: write [i j], not [i;j].

Write each line so its grouping is easy to see, and avoid spaces that aren’t needed. An operand often reads best touching its operator, with parentheses around a compound operand, as in ⌊@(10×)x and (≢⊢)⌸x. Spacing also makes a run into an operand: after a dyadic operator and a space, the whole next run is the operand, as in ⌊@ 10× x. When both operands are compound, parentheses around each show the grouping: (•ucs 9)@(⍸tabs)⍵. An operand keeps its parentheses when a left argument comes before it, as in tree (⍺⍢)/ next. A literal operand that would run into a vector argument needs the argument in touching brackets: f⍤1[2 3]. For a simple numeric argument, f⍣¯2⊢5 can read more clearly than f⍣¯2(5). Judge each line by how it reads, and report any case where none of these forms works.

Prefer a train to naming an argument twice: +/÷≢ v, not (+/v)÷≢v.

fib10 means fib 10, not an identifier (which would be fib_10). Write calls that way where it’s shorter and needs no new parentheses. Keep the space where the literal would join the next part of the expression. A left argument must be in the same run as its function: prefer 32•float∞ to 32 •float ∞. 32 •float∞ gives the two items 32 64ₓ, because •float∞ alone gives ∞’s width. Between two names, keep both spaces: x f 5, not x f5.

Read a selection with ⌷, as in [⍋v]⌷v. Write one literal position with a subscript, as in v₁ and v₁←0. Use . for other assignment targets, such as v.[i]←0, for record fields, such as T.name, and wherever ⌷ would be longer. For a target that is a cross product of positions, write ⌷ with a spaced ←, as in [1 3;0 5]⌷a +←10. . pairs its index arrays instead: a.[1 3;0 5] selects only the items at positions 1 0 and 3 5.

Positions and counts are exact, while bare numbers are floats. Arithmetic that mixes the two gives floats. A write that mixes them leaves the array in mixed storage. Write ⍳10ₓ or ⍳≢x for exact positions, not ⍳10. Seed an array that will hold positions or counts with exact numbers, as in n⍴¯1ₓ, and start a counter at 0ₓ. On positions, write ≥i, i+1ₓ and 2ₓ×i, not i+1 and 2×i. For an exact v, +/÷≢ v gives an exact rational. Write +/÷1×≢ v where a float is meant. Where a map’s branches give Booleans and numbers, convert the Boolean: +b gives exact integers, and 1×b gives floats.