Higher-order functions homework in SML

Question

I'm having trouble writing a function that will take a list of functions and an argument, then call each function with the passed argument, returning a list of the results of the calls. Example: build [f, g, h] 2 would return this, but with the functions called and the results instead of the call: [f(2), g(2), h(2)] Using SML/NJ, by the way.

First I tried many variants of this pattern:

fun build functions TheArgument = if functions = [] then [] else
    [hd(functions) TheArgument] @ build tl(functions) TheArgument;

But it gave the following error:

stdIn:2.9-2.36 Error: operator is not a function [equality type required]
  operator: ''Z
  in expression:
    (hd functions) TheArgument
stdIn:1.10-2.70 Error: case object and rules don't agree [tycon mismatch]
  rule domain: ''Z list * 'Y
  object: ('X list -> 'X list) * 'W
  in expression:
    (case (arg,arg)
      of (functions,TheArgument) =>
           if functions = nil then nil else (<exp> :: <exp>) @ <exp> <exp>)

Finally, I gave up and tried to do some research. I found the following question: Higher Order Functions in SML/NJ

I tried redefining it as this:

fun build [] argument = []
|   build f::rest argument = [f(argument)] @ build rest argument;

But then the compiler spits this:

stdIn:2.14-2.16 Error: infix operator "::" used without "op" in fun dec
stdIn:1.10-2.67 Error: clauses don't all have same number of patterns
stdIn:2.14-2.16 Error: data constructor :: used without argument in pattern
stdIn:1.10-2.67 Error: types of rules don't agree [tycon mismatch]
  earlier rule(s): 'Z list * 'Y -> 'X list
  this rule: ('W -> 'V) * 'U * 'T * 'W -> 'V list
  in rule:
    (f,_,rest,argument) => (f argument :: nil) @ (build rest) argument

What am I doing wrong?

I'm at a serious loss here, I can deal with cryptic Java/C error messages, but this is just too alien to me.

p.s.: The function cannot be called via build(functions, argument), it NEEDS to be two arguments and not a tuple of 2 arguments.

Note that instead of using append (@) to add one element infront of a list i kind, you should use cons (::) instead. In the general case you should always avoid using append when possible. It is especially important if the right hand list is big, due to the uncured running time. In such a case you should produce the list in reverse order, and then reverse it when you are done. — Jesper.Reenberg
The only thing you forgot in your final version was to put parenthesis around the pattern f::rest. Without those your second equation for build has four arguments: f, ::, rest, argument (instead of two, as indicated by the first equation). And that is exactly what the compiler was telling you (in his on words) ;) — chris

vishal vishal · Accepted Answer · 2017-09-21T00:56:37

stdIn:2.14-2.16 Error: infix operator "::" used without "op" in fun dec

this above error is because of you have not used brakets outside of f::rest so this can be resolved as

fun build [] argument = []
|   build (f::rest) argument = [f(argument)] @ build rest argument;

it sml interpreter is not able understand that this is list hence ...

Higher-order functions homework in SML

2 Answers