TextIO.inputN dramatically worse than TextIO.input

In the file at


https://github.com/HOL-Theorem-Prover/HOL/blob/40158b75c83806e2901edf0037677582098ec5a3/tools/quote-filter/mlton-quote-filter.sml

there is some code for implementing a simple Unix-style filter, based on some
logic in an mllex lexer (that’s what’s hiding in the structure “filter”).

On lines 13 and 21 I set the lexer up with an input function that is supposed to
return some of the input.  For the interactive version (line 21), using
TextIO.input makes sense because you want to echo (filtered) user input back as
soon as possible.  But in the non-interactive version, I initially thought I’d
use TextIO.inputN.

I was surprised to find that it actually performed 4 times worse on an input
file of size 10MB, taking about 80s instead of 20s.

The fact that either version is taking 20s is pretty painful in itself (a
Poly/ML version of this program does the same input in about 4s).

I’m running on

>    Linux telemachus 3.2.0-75-generic #110-Ubuntu SMP Tue Dec 16 19:11:55 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

with mlton version 20100608.

Should I expect to see this discrepancy?  And what is the point of inputN if so?

Michael

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