sml - ML. How to correctly work with sequences -

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so have "new" 2 way sequence:

datatype direction = | forward;  datatype 'a bseq =   bnil                  | bcons of 'a * (direction -> 'a bseq);

and need function seq2bseq : 'a seq -> 'a seq -> 'a bseq "appends" 2 regular sequences 1 so:

if sequp 0 1 2 3 4 ... , seqdown -1 -2 -3 -4 ... seq2bseq create .. -4 -3 -2 -1 0 1 2 3 4 .. . in other words starter element first of sequp(0) if move first element of seqdown(-1) , 2 second of sequp(1) if move forward. far wrote following:

fun appendq (nil, yq) = yq         | appendq (cons(x,xf), yq) = cons(x,fn()=>appendq(xf(),yq)); fun seq2bseq (downseq) (upseq) =             bcons(head(upseq), fn (forward) => seq2bseq appendq(head(upseq), downseq) tail(upseq)                              | (back) => seq2bseq tail(downseq) appendq(head(downseq), upseq) );

for following errors:

stdin:28.101-28.153 error: operator , operand don't agree [tycon mismatch]   operator domain: 'z seq * 'z seq -> 'z seq   operand:         'y seq -> 'y seq   in expression:     seq2bseq tail stdin:27.5-28.155 error: right-hand-side of clause doesn't agree function result type [tycon mismatch]   expression:  _ seq -> _ bseq   result type:  _ * _ -> ('z seq -> 'z seq) -> _ seq -> _   in declaration:     seq2bseq = (fn arg => (fn <pat> => <exp>))

i can't figure out what's wrong(:/). help! thanks!

edit: working(?) code at: http://www.beetxt.com/mkx/.

your type errors appear coming lack of parenthesization.

if have functions foo , bar, , want call foo on result of calling bar on value baz, need write foo (bar baz) or foo (bar (baz)), if prefer.

writing foo bar(baz) cause foo called argument bar, not type-check.

try:

fun seq2bseq (downseq) (upseq) =         bcons(head(upseq),           fn forward => seq2bseq (appendq(head(upseq), downseq)) (tail(upseq))             | => seq2bseq (tail(downseq)) (appendq(head(downseq), upseq))          )

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