CODEWITHSUNDEEP
smlsmlnj
700resu
700resu

Reputation: 259

Why is there type mismatch of operator and operand?

Why is there a tycon mismatch operator and operand do not agree error? Any Suggestion for a solution?

fun reve (x:string) = implode o rev o explode x

Upvotes: 0

Views: 967

Answers (1)

Sebastian Paaske Tørholm
Sebastian Paaske Tørholm

Reputation: 50858

The problem lies in, that function application binds stronger than o.

That is, it is interpreted as:

fun reve x = implode o rev o (explode x);

Since explode x is a char list and not a function, this will fail.

You can fix this by placing your parentheses properly

fun reve x = (implode o rev o explode) x;

Or writing it in point-free notation:

val reve = implode o rev o explode

It's also possible to define a right-associative function application operator, usually called $, which does what you want without parentheses:

(* Right-associative function application *)
infixr $
fun f $ x = f x;

fun reve x = implode o rev o explode $ x

Upvotes: 2

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